Valentine Hrytsenko, the marketing chief of a Ukrainian security-system company, was fleeing Kyiv with his one-month-old in the first hours of the war when he got the Facebook message: What did his company’s coders know about air-raid sirens?

The head of a Ukrainian app developer, Stepan Tanasiychuk, had friends who’d struggled to hear the haunting alarms triggered by civil-defense workers anytime the military radioed in missile strikes. The Cold War relics weren’t audible in bomb shelters or outside cities - a critical flaw for Ukrainians desperate to evade surprise attacks.

But Hrytsenko’s company, Ajax Systems, had for years built apps that could alert a person’s smartphone even in sleep or silent mode. Tanasiychuk knew it, because he owned one of their home-security systems himself.

Working from homes and shelters, a hastily assembled team of programmers from both companies built the siren app “Air Alert” in a single sleepless day. And every day since, they have rolled out updates to what has become the most-downloaded app in all of Ukraine. More than 4 million people use it today.

“Yesterday, some soldiers ... told us this app had saved their lives,” Hrytsenko said. “We feel very proud ... to help the country to fight.”

In peacetime, the programmers of Ukraine’s tech scene crafted the consumer software that powered homegrown start-ups and some of Silicon Valley’s biggest names.

Now, they build apps of war - an unprecedented digital infrastructure designed for both front-line combat and the realities of life under siege.

There are glossy online tools for rallying anti-Kremlin protests and documenting war crimes. There are apps for coordinating supply deliveries, finding evacuation routes and contributing to cyberattacks against Russian military websites.

There’s even an app people can use to report the movements of Russian troops, sending location-tagged videos directly to Ukrainian intelligence. The country’s minister of digital transformation, Mykhailo Fedorov, told The Washington Post they’re getting tens of thousands of reports a day.

Ukrainians have skillfully used social media to neutralize propaganda in Russia, rally spirits at home and mobilize antiwar sentiment around the world.

But the apps show how the invaded country has weaponized the internet’s power in a subtler way, expanding the reach of strained civilian resources and crowdsourcing the nation’s urban defense.

The displaced Ukrainians who built the apps said they’re realistic about the impact they’ll have in a devastating war. But they said they are pouring their lives into the tools on the chance they could help stop the carnage, working even as they stock up on body armor, uproot their families and dig in for the battles ahead.

The developers say Ukraine was primed for this kind of resistance. Boosted by project-outsourcing budgets from the West, the country’s tech sector has become a digital juggernaut, with thousands working for homegrown start-ups and American tech giants including Google, Oracle, Snap and Amazon’s Ring.

Many of the software designers, engineers and hackers who called Ukraine home have seen their traditional work disrupted and been thrown into lives of uncertainty, fear and national pride. Nearly all of them still have functioning internet.

In Russia, independent media has been squashed, dissenting voices arrested and the nation’s internet clogged with propaganda and conspiracy theories about a war the Kremlin won’t let people call a war.

But the Ukrainian government has welcomed this freewheeling style of technical innovation at its highest levels. In early 2020, its Ministry of Digital Transformation launched an app, “Diia” or “Action” (Дія), that worked like a digital driver’s license and included links to public services, from coronavirus-vaccination certificates to construction permits.

Its leader, the 31-year-old Fedorov, had previously launched an online marketing agency that ran Ukrainian President Volodymyr Zelensky’s winning “e-Zelensky” election campaign. Since the war began, Fedorov has used his visibly brash social media persona to pressure tech companies in the West to defy the Russian state.

The ministry’s app has evolved, too, from a stodgy public-service network into a full-scale companion for Ukrainians at war. The app now includes remote-job listings for Ukrainians out of work; a portal of cash payouts for citizens fleeing from combat; and even a set of video-learning lessons and math classes for children stranded away from school.

The app has become a lifeline - and a popular one, ranking within the top three most-downloaded apps in Ukraine all month, data from the analytics firm Sensor Tower show.

But some of its most prominent features are unmistakably militaristic. The app’s developers have started allowing ordinary Ukrainians to submit location-tagged photos and videos of Russian military sightings - as well as tips on “suspicious” people who might be invaders or saboteurs. The data, Fedorov told The Post in an interview, are aggregated onto a map visible to Ukrainian intelligence officials working on defense and counterstrikes.

Ministry officials have also begun pushing the app’s capabilities into controversial frontiers. Fedorov said on social media this week that his team, which once used facial recognition scans to verify Ukrainians’ identities for government services, has started adapting the face-scanning technology to identify the faces of dead Russian soldiers.

A ministry official told The Post last week that the project is “in very early development” and would likely rely on software offered by the facial recognition firm Clearview AI, which has been criticized by international governments for fueling its database with billions of people’s face photos taken from social media and other websites.

Ukrainian officials say the technology would help refute the Kremlin’s claims that only a small number of soldiers have been killed during what it has called a limited military operation. Fedorov told Reuters that Ukrainian authorities had already used the dead soldiers’ identifications to contact their relatives back in Russia. Those claims could not be independently confirmed.

Officials in Federov’s agency are also working on systems to mass-dial Russian phone numbers to share the grisly truth about the war in hopes of spurring antiwar dissent. “We have all changed. We started doing things we couldn’t even imagine a month ago,” Fedorov said in a roughly translated Instagram post Wednesday. “Thank you to everyone for the fight.”

For people wanting to report Russian military locations and behaviors without the Diia app, they can send information to eVorog, a ministry chatbot on the messaging service Telegram. After verifying that the sender is not Russian, the chatbot asks for the exact location of the military “equipment or occupiers” alongside a photo or video of the scene. That information, the ministry said, is then sent to the Ukrainian military to “quickly repel the enemy.”

Ukraine’s Security Service, its top law-enforcement agency, runs a separate Telegram bot, @stop_russian_war_bot, that allows people to submit sightings of “suspicious” people or vehicles. Images promoting the tool on Facebook, Instagram and Telegram show a Russian tank in the crosshairs, a QR code for easy downloading and a question in Ukrainian: “Did you see the enemy?”

Many other Ukrainian agencies have rolled out their own online tools. The country’s Office of the Attorney General has created a website for reporting war crimes that allows anyone to submit photos, videos and geolocation data for further investigation of a long list of potential horrors.

The office uses the data and other reports to offer daily estimates of possible war crimes. A checkbox on the reporting site lists “torture (beatings, rape, mutilation),” “murders, injuries to medical personnel” and “use by the occupier of civilian clothes.”

The Ukrainian Information Ministry’s Centre for Strategic Communications and Information Security has used Google forms to organize antiwar protests in 18 cities around the world, with questions like “How many people could you theoretically bring to a rally?”

And Ukraine’s International Legion of Territorial Defense, a foreign legion founded last month to recruit volunteer fighters from around the world, created a glossy website with tips for traveling to the battlefield: “It is recommended, if available, to bring your military kit ... [including] helmet [and] body armor.”

An interactive map on the site allows visitors to see how many volunteers traveled from each country. Russia, in red, is shown as “negative 14,000” - Ukraine’s unconfirmed assessment of the number of Russian soldiers killed in action.

Official government accounts have shared such numbers constantly on social media. On Instagram, Ukraine’s Ministry of Foreign Affairs posts daily tallies of what the Russian military has lost, including 1,578 armored vehicles, 517 tanks and 42 military drones.

The country has also used Telegram to give directions to hundreds of thousands of “IT Army” volunteers on which Russian websites to overload. Ukrainian fighters have also used social apps to exact more direct punishment; a Wall Street Journal journalist said Ukrainian soldiers and Territorial Defense volunteers have used the messaging app Viber to direct artillery fire at Russian troops.

Beyond the government-led efforts, Ukrainian tech companies have unveiled a number of their own wartime tools. There’s Prykhystok, a site for coordinating room-sharing arrangements to house Ukrainian refugees escaping active war zones.

There’s an evacuation site from the nonprofit UkraineNow that connects volunteer drivers with people looking to hitch a ride. Another site, Pomich, made by the Ukrainian freight-tech start-up Cargofy, helps link up truck owners with people seeking to move food and humanitarian supplies.

There’s a wartime job board for online-work requests: web developers, graphic designers, language translators and online anti-propaganda volunteers. And then there are apps like Play for Ukraine, an online puzzle game that, in the background, uses the player’s internet connection to blast thousands of online requests at Russian websites in hopes of helping take them down.

Most of the efforts are untested, and each carries their own risks in a war zone where thousands have already been killed. One refugee-housing site, for instance, faced criticism that a lack of security checks for hosts could mean people might end up staying somewhere dangerous. (The site has since relaunched with a more aggressive process for verifying hosts’ identities.)

But the Russian invasion has clearly given Ukrainian tech workers a new calling, and several said they were emboldened by the idea that they were building tools for the public good.

Tanasiychuk, founder of the development firm Stfalcon that helped with “Air Alert,” had previously led a team doing outsourced app projects for Western companies looking to plan bus routes and sell concert tickets. The closest they’d come to the military action was a cheeky smartphone game released shortly after Russia invaded Crimea, The Last Outpost, centered on a soldier defending his homeland against waves of invaders in Russian hats.

Developing and updating “Air Alert,” he said, often felt like a chaotic jumble of late-night calls and Telegram chats. To get it to work, the private-industry coders had to work with federal authorities and local units for emergency services and civil defense. They also had to design a digital system so the operators of the old-school analog sirens could easily ping millions of phones; now, when a possible airstrike is reported, they hit two buttons, instead of one.

The coders weren’t sure how it’d be received, writing in an announcement: “The app was developed in an emergency during one day, so there may be some minor problems.” But within a day of its debut on the app stores, it was downloaded more than 100,000 times. Google has since made the alert notifications available for all users in Ukraine.

The air-raid warnings have become a powerful symbol of a nation on edge: In one of his daily video reports this week posted to Telegram, Zelensky held up his phone while a siren played, saying, “We hear this for hours, days, weeks.”

But the app’s developers also feel it is a symbol of what a driven team can accomplish. Even as they scramble to keep their day jobs running, they have continued to roll out free, once-a-day updates fixing bugs and adding new monitored regions. Upcoming versions will include new types of alarms for shelling, street fighting and chemical-weapons attacks.

“It’s the best thing me and my team did in our lives,” Tanasiychuk said. “Each team member, they worked also for their parents, their neighbors, their relatives. They were worried for them ... and if you have this feeling, you can break mountains.”

Cat Zakrzewski and Rachel Lerman contributed to this report.