A US military analyst, Bradley Manning, has been arrested on suspicion of leaking classified combat video and documents to a whistle-blower website.
Specialist Manning, 22, was detained during a tour of duty in Iraq, and is being held in Kuwait pending further investigations.
The WikiLeaks website posted a video which it says shows the US military shooting civilians in Baghdad in 2007.
It has not confirmed Spc Manning as its source for the helicopter footage.
News of his arrest first broke on the Wired.com website.
A former hacker said he had turned the analyst in out of concern for US national security.
‘Boasting’ about leaks
In a statement, the US army in Iraq said Spc Manning was “placed in pre-trial confinement for allegedly releasing classified information”.
WikiLeaks’ organisers said they were given the footage, which they said came from cameras on US Apache helicopters.
They said they decrypted it, but would not reveal who gave it to them.