The classified task force investigated the Strike and acknowledged that four civilians were killed, but it also concluded that there had been no wrongdoing by the Special Operations unit. In October 2019, the task force sent its findings to the Central Command headquarters in Tampa, Fla.

But officials at Central Command did not follow up and failed to remind a subordinate military headquarters in Baghdad to do so, in what Capt. Bill Urban, a Central Command spokesman, described as “an administrative oversight.” As a result, senior military officials in Iraq and Florida never reviewed the strike, and the investigation technically remained open until the Times investigation.

Mr. Austin, who became defense secretary this year, received a classified briefing this month about the strike and the military’s handling of it from General McKenzie, who oversaw the air war in Syria.

The Times investigation showed that the death toll from the strike — 80 people — was almost immediately apparent to military officials. A legal officer flagged the bombing as a possible war crime that required an investigation. But at nearly every step, the military made moves that concealed the catastrophic strike. 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.

In an email to the Senate Armed Services Committee this spring, the legal officer who witnessed the strike warned that “senior ranking U.S. military officials intentionally and systematically circumvented the deliberate strike process,” and that there was a good chance that “the highest levels of government remained unaware of what was happening on the ground.”

A spokesman for the Armed Services Committee, Chip Unruh, said that the panel “remains actively engaged and continues to look at the matter.” Representative Adam Smith, Democrat of Washington and the chairman of the House Armed Services Committee, announced this month that his panel would also investigate the strike and the military’s handling of it.

The Times investigation found that the bombing by Air Force F-15E attack jets had been called in by Task Force 9, a unit largely made up of the U.S. Army’s elite Delta Force. The task force was in charge of ground operations in Syria, working closely with Syrian Kurdish and Arab militia. Military personnel who spoke to The Times said the secretive task force circumvented oversight by claiming that a vast majority of its strikes required immediate action to protect allied troops from imminent threat. Often, military officers said, no such threat was present.