“From my human point of view, I couldn’t believe it would be so easy to wreak that kind of havoc on the city, when they can’t see what is really going on there," Mr. Schmitt said in an interview from Germany on Monday night.

The Web site also posted a 17-minute edited version, which proved to be much more widely viewed on YouTube than the full version. Critics contend that the shorter video was misleading because it did not make clear that the attacks took place amid clashes in the neighborhood and that one of the men was carrying a rocket-propelled grenade.

By releasing such a graphic video, which a media organization had tried in vain to get through traditional channels, WikiLeaks has inserted itself in the national discussion about the role of journalism in the digital age. Where judges and plaintiffs could once stop or delay publication with a court order, WikiLeaks exists in a digital sphere in which information becomes instantly available.

“The most significant thing about the release of the Baghdad video is that several million more people are on the same page,” with knowledge of WikiLeaks, said Lisa Lynch, an assistant professor of journalism at Concordia University in Montreal, who recently published a paper about the site. “It is amazing that outside of the conventional channels of information something like this can happen.”

Reuters had tried for two and a half years through the Freedom of Information Act to obtain the Iraq video, to no avail. WikiLeaks, as always, refuses to say how it obtained the video, and credits only “our courageous source.”

Mr. Assange said “research institutions” offered to help decrypt the Army video, but he declined to detail how they went about it. After decrypting the attack video, WikiLeaks in concert with an Icelandic television channel sent two people to Baghdad last weekend to gather information about the killings, at a cost of $50,000, the site said.

Image In July 2007, a crowd gathered at the scene of an airstrike in Baghdad that killed 12 people, including two Reuters employees. Credit... Ali Abbas/European Pressphoto Agency

David Schlesinger, Reuters editor in chief, said Tuesday that the video was disturbing to watch “but also important to watch.” He said he hoped to meet with the Pentagon “to press the need to learn lessons from this tragedy.”