WikiLeaks has posted a video on its website which it claims shows the killing of civilians by the US military in Baghdad in 2007.
The website's organisers say they were given the footage, which they say comes from cameras on US Apache helicopters.
They say they decrypted it, but would not reveal who gave it to them.
The WikiLeaks site campaigns for freedom of information and posts leaked documents online. So far there has been no official Pentagon response.
However, Reuters and the Associated Press have quoted unnamed US military officials as confirming the video was genuine.
The video, released on Monday, is of high quality and appears to be authentic, the BBC's Adam Brookes in Washington says.
It is accompanied by a recording of the pilots' radio transmissions and those of US troops on the ground.
The video shows a street in Baghdad and a group of about eight people, whom the helicopter pilots identify as armed insurgents.
The transmissions says of one of the individuals: "He's got an RPG [rocket-propelled grenade]. I'm going to fire."
After a voice on the transmission urges the pilot to "light 'em all up", the individuals on the street are shot by the gunship's cannon.
A few minutes later a van drives to the scene, and its occupants appear to start picking up a wounded person.
It, too, is fired upon. Altogether, around 12 people die.
The transmission continues: "Looks like we've got some slight movement from the van that was engaged. Looks like a kid."
US soldiers on the ground establish there are two child casualties and agree to take them to a hospital, according to the transmission.
"Well it's their fault for bringing their kids into a battle," says a voice.
Two journalists working for Reuters were killed on the day the incident took place in July 2007.
A spokeswoman for the news agency said they were not sure if the individuals in the footage included those two Reuters journalists.
WikiLeaks has published a statement from Reuters news editor-in-chief David Schlesinger saying that the video was "graphic evidence of the dangers involved in war journalism and the tragedies that can result".
At the time, the US military said the helicopters were engaged in combat operations against a hostile force.
WikiLeaks said the video demonstrated that civilians had died in the incident, and that the US military's rules of engagement were flawed.
The website's organisers complained recently of coming under surveillance by the US government, and of harassment by other governments, ostensibly for their role in posting leaked documents on sensitive subjects.