Three days after the Oct. 7 terrorist attack, one of Hamas’s top political leaders put out a call for a new front in the group’s conflict with Israel - a fight to be waged not with bullets, but with dollars.

“This is financial jihad,” Khaled Mashal, the group’s former political chief, declared in a speech disseminated over social media. He urged supporters worldwide to give “aid, money and all that you have,” adding, “don’t let your brothers down.”

Within days, a torrent of cash began pouring into accounts set up to help Gazans, much of it from people moved by images of victims of Israeli airstrikes and genuinely wanting to help. But also answering the call were groups with years of experience in delivering precisely the kind of jihad Hamas’s leader envisioned.

Across the Middle East and Europe, the Gaza conflict re-energized old fundraising networks with ties to militant Islamist groups and causes, including groups accused of raising money in the past for al-Qaida and the Taliban as well as Hamas’s military wing, current and former U.S. counterterrorism officials say.

The groups helped raised millions of dollars practically overnight, using crowdfunding campaigns on social media that were built around photos and videos depicting the suffering of Gazan civilians. Some of the money was ultimately deposited in Hamas-controlled accounts using a range of methods, including cryptocurrency and informal cash-transfer networks used in Middle Eastern countries for centuries, the officials said.

Among those backing Hamas crowdfunding campaigns, officials said, are groups such as Kuwait’s Revival of Islamic Heritage Society, which has been accused by both the United Nations and U.S. government of raising money for al-Qaida; as well as Europe-based charities with ties to the Union of Good, an umbrella organization that U.S. officials say was created by Hamas leaders and supporters to facilitate the transfer of money to the group. Both groups have denied supporting or advocating terrorism. Hamas’s founding charter calls for the destruction of Israel, and the group is regarded as a terrorist organization by the United States and the European Union.

While the key fundraisers have been active in some cases for decades, their tactics have been perfected in recent years as the groups learned to combine the vast reach of platforms such Facebook and X, formerly known as Twitter, with the emotional power of an international tragedy. Militant Islamist groups and their supporters embraced the use of crowdfunding on social media after the start of the Arab Spring uprisings, particularly in Syria. Some used the technique to solicit money in the aftermath of last year’s devastating earthquake in Turkey and Syria.

“There is a legitimate, tremendous need in Gaza right now, and it’s very easy for someone to put up a website with a photo of an injured baby,” said Matthew Levitt, a former Treasury Department counterterrorism official now at the Washington Institute for Near East Policy, a think tank. “This is a real opportunity if you’re a Hamas fundraiser.”

It is difficult to obtain precise figures for the amount of money that Hamas has generated through crowdfunding efforts in the months since the conflict began. Israeli government officials in interviews put the total at about $200 million, a sum they acknowledge includes money raised by legitimate charities seeking to help the millions of Gazans in dire need of food, medicine and shelter.

Israeli officials say Hamas uses some of the money to pay the salaries of its fighters and to finance the group’s political and influence operations abroad. In the future, they say, surviving Hamas leaders could use the funds to replenish the group’s stockpile of rockets and other weapons.

U.S., European and Middle Eastern officials all assess that crowdfunding has become a particularly important source of revenue for Hamas since the conflict began with the Oct. 7 terrorist attack on Israel. Since the start of the Israeli ground offensive, Hamas, the governing authority for the Gaza Strip, has been largely unable to collect taxes and fees from Gazans, its biggest single income generator.

Travel bans for Gazans and banking restrictions imposed by the United States and other Western governments have made it difficult for Hamas to collect financial support from Iran and other foreign backers, or to cash in on its estimated $500 million investment portfolio - a substantial resource on paper that has offered only minimal practical benefit for the group during the current crisis, since the portfolio consists mainly of assets that can’t be quickly converted to cash, such as mines, farms and real estate, former U.S. officials said.

Instead, Hamas has been compelled to rely increasingly on crowdfunding efforts, with substantial assistance from allies and longtime supporters of Islamist causes, current and former U.S. officials said.

One example is a Kuwait-based initiative that was launched Oct. 9 - two days after the terrorist attack and well before Israel’s ground offensive began. A crowdfunding operation that called itself the “Mobilization for Palestine Campaign” appeared online, soliciting donations for “affected families” in Gaza. It offered donors multiple options, including payment by ApplePay, Visa and Mastercard.

The campaign generated $8.3 million in contributions in just the first three days, according to a review by Kharon, a Los Angeles-based private intelligence company.

Among the listed sponsors of the campaign was the Revival of Islamic Heritage Society. The group, also based in Kuwait, was added to a sanctions list by the Treasury Department in 2008 for allegedly providing financial support to al-Qaida and other terrorist groups. The United Nations in 2011 similarly accused the group’s Pakistan and Afghanistan branches of aiding al-Qaida leader Osama bin Laden and couriering money and messages to al-Qaida members in Pakistan. In 2013, the group participated in a Kuwaiti-led campaign to raise money for rockets and other weapons for 12,000 Islamist militants fighting on the rebel side in Syria’s civil war.

“This one got our attention because of the organizations involved that were able to raise $8 million in less than a week,” said Mark Nakhla, a former Treasury sanctions official who now leads investigations of illicit financial networks for Kharon. “This is ongoing, and new campaigns are emerging on a weekly basis.”

Emails and voice messages requesting comment from Society officials were not returned. In previous statements, the organization has rejected accusations that its fundraising benefited violent extremists. “The society strongly denies any connection to terrorist acts, or factions that use them, be it Osama bin Laden and al-Qaida or any other,” the group said in a 2001 statement.

The Biden administration is raising concerns with Western governments over similar fundraising by European groups with alleged ties to militant Islam. The Treasury Department in November expanded its sanctions list to include Humani’Terre, a French nonprofit that runs crowdfunding campaigns on social media in support of Gaza. Treasury documents assert that Humani’Terre is in fact a rebranded version of another charity: the Paris-based Committee for Charity and Support for the Palestinians, or CBSP by its French initials. The two organizations share the same address and telephone number.

The CBSP has been under U.S. sanctions since 2003 over allegations that its serves as a primary fundraising arm of Hamas, although the group has vigorously denied supporting terrorist groups or sending money to Hamas’s militant wing. Treasury documents announcing the sanctions accused CBSP leaders of collecting “large amounts of money from mosques and Islamic centers, which it then transfers to sub-organizations of Hamas.”

A statement posted online by Humani’Terre asserts that the group’s main website has been disabled by unnamed outsiders, but says it continues to raise funds for Gaza relief. A person who answered the phone at the Humani’Terre office said the group would not comment to journalists. An email message requesting an interview was not returned. CBSP has repeatedly contested claims that it has supported terrorism, including in successful civil defamation cases against French organizations that accused the group of knowingly supporting terrorist groups or providing money to families of suicide bombers. A CBSP spokesman, in a 2010 interview with French journalists, said the sanctions were the product of a U.S. mind-set after the Sept. 11, 2001, attacks that regarded “each humanitarian organization serving the Palestinians as a terrorist movement.”

Since Oct. 7, top Treasury officials have made repeated visits to European and Middle Eastern capitals to coordinate efforts to halt fundraising for Hamas while allowing money to flow to legitimate relief organizations that are helping alleviate suffering in Gaza. Much of the diplomacy has focused on Turkey, whose government has historically supported Hamas and allowed the group to set up front companies and pro-Hamas charities, several of which are now under U.S. sanctions.

A Treasury official described the exchanges with Turkish officials as “tough conversations,” but said the Biden administration is succeeding in blocking Hamas’s ability to access foreign bank accounts. International banking companies are generally wary of doing business with groups that are on the U.S. blacklist, and “we are pushing them to do a better job” at detecting and halting flows of cash to Hamas, said the official, who spoke on the condition of anonymity to discuss politically sensitive deliberations with foreign governments and international banks.

In the last two years, the Treasury Department has imposed sanctions against more than 40 companies and individuals with alleged links to Hamas’s financial networks and foreign investments. Since Oct. 7, U.S. officials have also met with business executives to encourage tougher measures to prevent Hamas from using American banking networks and credit-card and payment services companies. The campaign to starve Hamas financially has been so aggressive that, in recent weeks, the Biden administration has taken steps to ensure that legitimate relief organizations can continue to access money to do work in Gaza, the official said.

“We are trying to be proscriptive in going after illegal charities while letting legitimate ones bring in aid,” the Treasury official said.

Yet, as current and former officials acknowledge, Hamas has found clever ways to get at some of the cash its supporters raise abroad. These include a mix of modern tactics and other methods that date back to the late Middle Ages.

Hamas is an experienced user of cryptocurrency and has frequently accepted transfers of digital funds - usually bitcoin - from overseas accounts to virtual “wallets” controlled by the group. Treasury records since 2020 identify more than 300 transactions involving transfers of cryptocurrency to Hamas accounts for “potential terrorist financing purposes.”

Such transactions typically involve relatively modest sums, current and former officials said. Treasury counterterrorism investigators, in a probe spanning several years, tracked a number of small transfers involving a single Gaza-based business called Buy Cash, a company offering virtual currency exchange and money transfer services to Palestinians. Some of the money that landed in the Buy Cash virtual wallet appears to have been gifts or loans to Hamas from individuals with links to other terrorist groups, Treasury officials said. For example, a transfer of $2,000 in 2019 was initiated by a known al-Qaida affiliate in Turkey. Another transfer in 2017 involved an account previously linked to the Islamic State, the officials said. Treasury officials imposed sanctions on Buy Cash on Oct. 18. Efforts to reach the business by email for comment were not successful.

Larger sums intended for Hamas are moved through currency exchanges in Lebanon, Syria and Turkey, as well through the Hawala system, an informal banking network native to South Asia and the Middle East, current and former officials say. Hawalas generally charge only small transaction fees - some Muslims view interest payments as haram, or forbidden - and they rely on a kind of Islamic honor code to transfer large sums of money, including between parties separated by great distances and international boundaries. The transactions are all but invisible to traditional banking systems as well as law enforcement agencies.

Since the 2000s, the exchange networks remain among the biggest obstacles to stemming the flow of donated money to terrorist groups, said Levitt, the former Treasury counterterrorism official. Hamas, like al-Qaida, has become particularly adept at using the system, and thus concealing much of its fundraising activity from the outside world.

“There are lots of ways,” Levitt said, “but for the big money, this is how it gets in.”

As an indication of the networks’ importance, the Israeli government promoted with great fanfare recent successful operations to kill two Hamas officials with deep ties to the group’s fundraising efforts, including informal exchanges. One was Hamas financier Subhi Ferwana, an official linked to Hamas’s currency exchange network who was killed, along with an unknown number of civilians, in an Israeli airstrike in southern Gaza in mid-December. Another was Saleh Arouri, the deputy Hamas chief killed in an Israeli strike on a Beirut suburb last week.

Arouri was described by Israeli officials as a principal planner of the Oct. 7 attacks. He had also been under U.S. sanctions since 2015 as a “key financier and financial facilitator” for the group’s military cells.