The U.S. military disclosed on Friday that an airstrike it carried out on an Islamic State checkpoint in Iraq likely killed at least four civilians, including a child.

The strike was launched by an A-10 attack plane on March 13 near Al Hatra, a city in northern Iraq about 70 miles southwest of Mosul. The strike had been approved at the Al Udeid Air Base in Qatar, but before the checkpoint was hit, a black Kia sedan and a GMC Suburban sports utility vehicle arrived on the scene and parked nearby for about 40 minutes, the investigation found. An air crew relayed this to the Combined Air Operations Center, which manages the air war from the Qatar base, but analysts assessed that the vehicles were legitimate targets.

“Post-strike imagery analysis of onboard weapons system video footage indicated that four additional personnel whose status was unknown, and previously undetected, exited the vehicles after the aircrews had released weapons on the target and immediately before the weapons impacted the area,” according to findings released Friday. “Video footage review indicates the aircrew had no opportunity to detect the presence of the likely civilians in the target area prior to weapons impact.”

The incident raises questions about how the U.S. military vets and carries out airstrikes against Islamic State militants, especially with limited intelligence available in the absence of U.S. ground forces in the region. The likely mistake was brought to the attention of the U.S. military within days by a woman who said her vehicle had been destroyed along with the second vehicle. Two women and three children burned to death, she alleged.

The investigation was carried out from April 22 to June 1. Video of the airstrike showed that six seconds prior to the weapons landing, four people got out of the SUV, including one who looked smaller than the other three — the likely child the military now acknowledges.

The U.S. military had been examining the strike site as a potential target for months, noting that ruins in the Al Hatra area were the site of a training camp for Islamic State recruits in the region. Seven other vehicles were seen passing through the checkpoint while U.S. planes flew overhead the day of the strike, but only the two hit stopped for a significant period of time, the investigation found.

The incident marks the second time that the United States has acknowledged killing civilians in its air war against the Islamic State and other militants in Iraq and Syria. In the first, two civilians were killed in an airstrike in Harim City, Syria, in November 2014, the Pentagon later acknowledged.

U.S. military officials have repeatedly stressed efforts to minimize and prevent civilian casualties. As of 4 p.m. Thursday, the U.S.-led military coalition has carried out 5,432 airstrikes in Iraq and 2,857 in Syria in the fight against the Islamic State. The United States dropped weapons in 6,571 of those cases, about 78 percent of the total airstrikes.

Military officials with U.S. Central Command said Friday that there are 12 other allegations of civilian casualties also under review. They include 10 in Iraq and two in Syria.