The 2019 attack on so-called Islamic State (IS) fighters killed 80 people – only 16 identified as militants.
An independent investigation into the strike was never conducted, according to the New York Times.
Officials admitted the bombing for the first time this week, but insisted it had been justified.
They said it was unclear if all of those killed aside from the 16 militants had been civilians.
The BBC has contacted the US military for details of the investigation on the strikes of 18 March 2019 in the town of Baghuz in eastern Syria – then the last stronghold of the so-called IS caliphate.
Three bombs were dropped by US jets on a large group of people, despite drone footage showing the presence of civilians, according to the New York Times. It said the strike was ordered by a classified US special operations unit tasked with ground operations in Syria.
Commanders ignored alarm expressed in the immediate aftermath and a subsequent investigation into the incident by the Defence Department’s inspector general was “stripped” of any mention of the strike, according to the newspaper. It added that no thorough independent investigation had ever happened.
“Leadership just seemed so set on burying this. No-one wanted anything to do with it,” Gene Tate, an official who worked on the case and who was subsequently sacked, told the newspaper.
When it finally acknowledged the strike following the newspaper’s investigation, US Central Command said the bombs had killed 16 fighters and four civilians. Of the remaining 60, the US military had concluded that they were not sure they were civilians “in part because women and children in the Islamic State sometimes took up arms”.
“In this case, we self-reported and investigated the strike according to our own evidence and take full responsibility for the unintended loss of life,” the New York Times cites Cpt Bill Urban, Central Command chief spokesman, as saying.
Kurdish-led Syrian Democratic Forces (SDF) – allied to the US – started the final ground attack on Baghuz at the start of March 2019.
The alliance was forced to slow its offensive after it emerged that a large number of civilians were also there, sheltering in buildings, tents and tunnels.
Many IS fighters stayed in their midst, putting up fierce resistance, deploying suicide bombers and car bombs.
The fall of Baghuz was a major moment in the fight against IS.
Who are IS?
The extremist Islamist group came to international prominence in 2014 when it seized large parts of Syria and Iraq.
IS imposed its brutal rule on almost eight million people, carried out countless atrocities, destroyed cultural heritage, and generated billions of dollars in revenue from oil, extortion, robbery and kidnapping.
After five years of fierce and bloody battles, local forces, backed by world powers, have driven IS out of all the territory it once controlled.
Last year, however, IS claimed to have carried out nearly 600 attacks in Syria and more than 1,400 in neighbouring Iraq.