Four Indiana National Guard soldiers killed in Afghanistan in late March were struck by an anti-tank mine left from Afghanistan’s long history of conflict, U.S. military officials said Thursday.

According to results released by the military in Kabul, investigators determined the mine was “likely left from previous wars and may have shifted during the recent rains.” The investigation was conducted by at least two explosive ordnance teams, officials said.

“There are no indications that the mine was deliberately set for or designed to target the soldiers,” the report concluded.

The incident highlights the continuing danger posed by the estimated 10 million to 15 million land mines and other pieces of unexploded ordnance remaining scattered throughout Afghanistan. According to the International Campaign to Ban Landmines, more than 150 people are injured or killed each month by mines and other leftover ordnance.

The new Afghanistan government joined the Mine Ban Treaty in 2002 and efforts are continuing to find and remove the ordnance.

Afghan provincial Gov. Mohammed Aman Hamini said shortly after the incident that it occurred in a desert area peppered with vehicle trails.

“It’s an old mine,” he told the Associated Press shortly after the incident. “There’s no traffic on the route they took, but the Russians used to use it because they were afraid of the main road.”

The four soldiers were members of Combined Joint Task Force Phoenix and were part of an American-Afghan patrol scouting possible locations for a shooting range in Logar province, about 25 miles south of Kabul, officials have said.

Task Force Phoenix oversees the training of the Afghan National Army.

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