In the last days of the battle against ISIS in Syria, when members of the once-fierce caliphate were cornered in a dirt field next to a town called Baghuz, a U.S. military drone circled high overhead, hunting for military targets. But it saw only a large crowd of women and children huddled against a riverbank.

Without warning, a U.S. F-15E attack jet streaked across the drone's high-definition field of vision and dropped a 500-pound bomb on the crowd. Then a jet dropped one 2,000-pound bomb, then another.

It was March 18, 2019. At the U.S. military's busy Combined Air Operations Center at Al Udeid Air Base in Qatar, uniformed personnel watching the live drone footage looked on in stunned disbelief.

"Who dropped that?" a confused analyst typed on a secure chat system being used by those monitoring the drone. Another responded, "We just dropped on 50 women and children."

An initial battle damage assessment quickly found that the number of dead was actually about 70.

The Baghuz strike was one of the largest civilian casualty incidents of the war against ISIS, but it has never been publicly acknowledged by the U.S. military. The details, reported here for the first time, show that the death toll was almost immediately apparent to military officials. A legal officer flagged the strike as a possible war crime that required an investigation. But at nearly every step, the military made moves that concealed the catastrophic strike. The death toll was downplayed. Reports were delayed, sanitized and classified. U.S.-led coalition forces bulldozed the blast site. And top leaders were not notified.

The Defense Department's independent inspector general began an inquiry, but the report containing its findings was stalled and stripped of any mention of the strike.

"Leadership just seemed so set on burying this," said Gene Tate, an evaluator who worked on the case for the Inspector General's Office and agreed to discuss aspects that were not classified.

Tate, a former Navy officer who had worked for years as a civilian analyst with the Defense Intelligence Agency and the National Counterterrorism Center before moving to the Inspector General's Office, said he criticized the lack of action and was eventually forced out of his job.

Details of the strikes were pieced together by the New York Times over months from confidential documents and descriptions of classified reports, as well as interviews with personnel directly involved and officials with top secret security clearances who discussed the incident on the condition that they not be named.

The Times investigation found that the bombing had been called in by a classified U.S. special operations unit, Task Force 9, in charge of ground operations in Syria. The task force operated in such secrecy that at times it did not inform even its own military partners of its actions. In the case of the Baghuz bombing, the U.S. Air Force command in Qatar had no idea the strike was coming.

After the strike, an alarmed Air Force intelligence officer called over an Air Force lawyer in charge of determining the legality of strikes. The lawyer ordered the F-15E squadron and the drone crew to preserve all video and other evidence. He reported the strike to his chain of command, saying it was a possible violation of the law of armed conflict — a war crime — and regulations required a thorough, independent investigation.

But it didn't happen.

This past week, after the Times sent its findings to U.S. Central Command, which oversaw the air war in Syria, the command acknowledged the strikes for the first time, saying 80 people were killed but the airstrikes were justified. It said the bombs killed 16 fighters and four civilians. As for the other 60 people killed, the command said it was not clear that they were civilians, in part because ISIS women and children sometimes took up arms.

The only assessment done immediately after the strike was performed by the same ground unit that ordered the strike. It determined that the bombing was lawful because it killed only a small number of civilians while targeting ISIS fighters in an attempt to protect coalition forces. Therefore, no formal war crime notification, criminal investigation or disciplinary action was warranted.

But the Air Force lawyer, Lt. Col. Dean Korsak, believed he had witnessed possible war crimes and repeatedly pressed his leadership and Air Force criminal investigators to act. When they did not, he alerted the Defense Department's independent inspector general. Two years after the strike, seeing no evidence that the watchdog agency was taking action, Korsak e-mailed the Senate Armed Services Committee.

A secret task force

The U.S. portrayed the air war against ISIS as the most precise and humane bombing campaign in its history. The military said every report of civilian casualties was investigated and the findings reported publicly, creating what the military called a model of accountability.

But the strikes on Baghuz tell a different story.

The details suggest that while the military put strict rules in place to protect civilians, the Special Operations task force repeatedly used other rules to skirt them.

Even in the extraordinary case of Baghuz — which would rank third on the military's worst civilian casualty events in Syria if 64 civilian deaths were acknowledged — regulations for reporting and investigating the potential crime were not followed, and no one was held accountable.

Nearly 1,000 strikes hit targets in Syria and Iraq in 2019, using 4,729 bombs and missiles. The official military tally of civilian dead for that entire year is only 22, and the strikes from March 18 are nowhere on the list.

Baghuz represented the end of a nearly five-year U.S.-led campaign to defeat ISIS in Syria. On the ground, Task Force 9 coordinated offensives and airstrikes. The unit included soldiers from the 5th Special Forces Group and the Army's elite Delta Force.

Over time, some officials overseeing the air campaign began to believe that the task force was systematically circumventing the safeguards created to limit civilian deaths.

The process was supposed to run through several checks and balances. Drones with high-definition cameras studied potential targets, sometimes for days or weeks. Analysts pored over intelligence data to differentiate combatants from civilians. And military lawyers were embedded to ensure that targeting complied with the law of armed conflict.

But there was a quick and easy way to skip much of that oversight: claiming imminent danger.