The statement said that “investigators could not conclusively determine how or when the women died, due to lack of forensic evidence” but that they had nonetheless “concluded that the women were accidentally killed as a result of the joint force firing at the men.”

“We deeply regret the outcome of this operation, accept responsibility for our actions that night, and know that this loss will be felt forever by the families,” said Brig. Gen. Eric Tremblay, a spokesman for the NATO command in Kabul.

The admission was an abrupt about-face. In a statement soon after the raid, NATO had claimed that its raiding party had stumbled upon the “bodies of three women who had been tied up, gagged and killed” and hidden in a room in the house. Military officials had also said later that the bodies showed signs of puncture and slashing wounds from a knife, and that the women appeared to have been killed several hours before the raid.

And in what would be a scandalous turn to the investigation, The Times of London reported Sunday night that Afghan investigators also determined that American forces not only killed the women but had also “dug bullets out of their victims’ bodies in the bloody aftermath” and then “washed the wounds with alcohol before lying to their superiors about what happened.”

A spokesman for the Afghan Interior Ministry, Zemary Bashary, said that he did not have any information about the Afghan investigation, which he said remained unfinished.

In an interview, a NATO official said the Afghan-led investigation team alerted American and NATO commanders that the inquiry had found signs of evidence tampering. A briefing was given by investigators to General McChrystal and other military officials in late March.

“There was evidence of tampering at the scene, walls being washed, bullets dug out of holes in the wall,” the NATO official said, adding that investigators “couldn’t find bullets from the wounds in the body.”