A defense official told the newspaper the authorities searched computers used by Manning, which had left evidence that he had downloaded the documents that provide a look at U.S. combat and frustrations in the war in Afghanistan - 76,000 of which were posted on WikiLeaks.org Sunday night, while 15,000 are being vetted by the group for future release.
Manning, 22, is the man charged earlier this month of illegally giving WikiLeaks a classified video showing a U.S. military helicopter firing on a group of people in Baghdad. Two Reuters journalists and seven others were killed in the incident.
Manning worked in the intelligence operations of the 2nd Brigade in Baghdad. He was supposed to be examining intelligence relevant to Iraq, but defense officials said he was using his "Top Secret/SCI" clearance to download classified documents.
More on Bradley Manning:
Suspicions have been on Manning ever since the Afghanistan logs were published Sunday. He is being detained in Kuwait, charged with "mishandling and leaking classified data." He was caught bragging about leaking the Iraq video on an internet chat. The chats also revealed Manning's frustrations at being "regularly ignored" at work.
"I've been isolated so long," he wrote. "I just wanted to be nice, and live a normal life ... but events kept forcing me to figure out ways to survive."
If convicted, Manning could be sentenced to a maximum 52 years in prison.
A U.S. official had earlier said everything he had seen on WikiLeaks could have been obtained by surfing a Defense Department intranet system known as the "SIPRNet," or Secret Internet Protocol Router Network, which Manning had access to.
It is not immediately clear the exact nature of the evidence linking Manning to the documents. The Federal Bureau of Investigation and the Justice Department have been brought in to aid the probe, which is lead by the Army Criminal Investigation Command.
More WikiLeaks Coverage:
WikiLeaks: We Don't Know Who Leaked Documents
Holder: DOJ Aiding Pentagon WikiLeaks Probe
WikiLeaks Puts Afghan Agents at Risk, U.S. Says
WikiLeaks Unlikely to Change Afghan War's Course
Pentagon: "Very Robust" Probe of WikiLeak Source
Leaked Docs Expose Afghan Failings, Plague Military