A U.S. airstrike in Syria last month hit a mosque and was “likely unlawful,” a leading human rights group said in a scathing report Tuesday.

“The U.S. seems to have gotten several things fundamentally wrong in this attack, and dozens of civilians paid the price,” Ole Solvang, deputy emergencies director at Human Rights Watch, said in a statement.

“The U.S. authorities need to figure out what went wrong, start doing their homework before they launch attacks and make sure it doesn’t happen again.”

At issue is a March 16 strike in Aleppo province. Immediately after the strike, Syrian activists reported that it hit a mosque, killing more than 40 people, mostly civilians.

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The Pentagon is investigating the strike. Officials have said the strike targeted an al Qaeda meeting.

While the Pentagon originally insisted the building was not a mosque, officials later said it could have been part of a larger “mosque complex.”

In its report, Human Rights Watch said the building was a well-known mosque.

“While the mosque did not have a minaret or a dome that would have been visible by aerial surveillance, local residents said that dozens, if not hundreds, of people were gathering in the building at prayer times,” the report says. “Aerial surveillance of the building should have shown this. Local residents also said that the mosque was well known and widely used by people in the area.”

The group based its report on phone interviews with four people who were at the building at the time of the strike; eight first responders, journalists and local residents who arrived at the scene shortly after; and two medical personnel who treated victims.

In addition to appearing to have “fundamentally misunderstood” the nature of the building, the report adds, the military appears to have “inadequately understood” the pattern of life in the village.

First, while U.S. officials say the strike happened after evening prayer ended, the report says the strike happened about 15 minutes before prayer started. Second, the report says, local residents said it was well known there are religious lectures in the building every Thursday between sunset prayer and evening prayer, about the time the strike happened.

Human Rights Watch said it has found no evidence to support the U.S. assertion that al Qaeda or any other armed group was meeting in the building. Several residents told the group that the Free Syrian Army, a rebel group considered moderate by the U.S., is in general control of the area, according to the report.

In conjunction with the report, Human Rights Watch also released video from Forensic Architecture, which conducted its own investigation into the strike. The videos show a before-and-after of the building using computer models, real video and photos. One of the clips taken two months before the strike shows a speaker used for call to prayer and a sign identifying the building as a mosque.

In a letter responding to the report, U.S. Central Command said a review reached a preliminary finding that the strike was lawful.

“We carefully review every allegation of civilian casualties that may have been caused by a coalition airstrike,” Staff Judge Advocate Col. Patrick Huston wrote in the April 14 letter, released Tuesday by Human Rights Watch.

“In this particular case, we also conducted a thorough examination of the classified intelligence used to inform the targeting decision, the classified intelligence that emerged following the strike. This comprehensive investigation reached the preliminary conclusion that the strike was lawful.”

The different conclusions reached by Human Rights Watch and the military could be because Human Rights Watch does not have access to the classified information, Huston added.

“Nevertheless, we always welcome additional information to assist our investigations,” he wrote, “and we will continue to carefully review the incident in light of your report.”