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.
Apparently, he boasted about his exploits.
The ex-hacker, Adrian Lamo, said Spc Manning “boasted” to him about passing the helicopter video to WikiLeaks.Mr Lamo said Spc Manning claimed to have leaked video footage showing an air strike in Afghanistan in July 2009. The local authorities said nearly 100 people were killed in the attack at Garani
Spc Manning also said he had passed on 260,000 classified US diplomatic cables.
However, WikiLeaks has said it was not sent the diplomatic cables.