Whistleblower website WikiLeaks is keeping quiet on how it managed to de-encrypt the graphic video of a US army assault in Baghdad in 2007 that left 12 people dead, including two Reuters news hacks.

The site asked for help from a supercomputer to decrypt the videos on Twitter and must have got some. The decrypted video has been viewed more than 2 million times on YouTube, and has been replayed hundreds of times in television news reports.

WikiLeaks labelled the video "Collateral Murder" and said it depicted "indiscriminate" and "unprovoked" slaying. It also posted a 17-minute edited version, which proved to be much more widely viewed on YouTube than the full version.

Critics claimed that the shorter video was misleading because it did not make clear that the attacks took place amid clashes in the neighbourhood and that one man was carrying a rocket-propelled grenade. Reuters had spent 2½ years without success trying to obtain the video through the Freedom of Information Act.