According to CNN, Manning is the prime suspect in the recent case, in which thousands of classified documents about the war in Afghanistan were leaked by WikiLeaks to publications including The New York Times and The Guardian.
Defense Secretary Robert M Gates denounced WikiLeaks on Thursday, saying that leaking the documents to the press endangered lives.
AP
Army Pfc. Bradley E. Manning is being held at the Quantico Marine Corps Base in northern Virginia, awaiting possible trial on 12 offenses.
"The battlefield consequences of the release of these documents are potentially severe and dangerous for our troops, our allies and Afghan partners, and may well damage our relationships and reputation in that key part of the world," Gates said, according to The New York Times.
Manning has already been charged with disseminating a classified video of an air strike in which U.S. military personnel are shown killing civilians, including Reuters journalists.
Now, authorities are investigating if Manning physically passed on discs that contained classified military information to somebody in the United States, CNN said.
Manning was processed at the Quantico detention facility in Virginia on Thursday. He may face a military judge in August, but the complexity of the case means there is a strong chance of a delay, CNN reported.
Manning, who is half-British, frequently complained about the Army via his Facebook page, The Daily Telegraph reports. His status updates included comments saying he was "beyond frustrated" and "Bradley Manning is not a piece of equipment."
He also quipped that the phrase "military intelligence" is an oxymoron.
The inquiry into the recent leak has been expanded to search for civilian accomplices, The New York Times reported.
Investigators are checking out Manning's social circle in Cambridge, Mass., in the belief that whoever leaked the documents may have had civilian help.
A computer hacker named Adrian Lamo told The New York Times that he believes WikiLeaks helped Manning with the technical aspect of downloading classified documents.
Lamo says he believes that WikiLeaks helped Manning devise encryption software that allowed him to e-mail classified data out of the military computer system.
Manning "was to a great extent manipulated by WikiLeaks." Lamo told The New York Times. Lamo admits he has no direct evidence to back his claim that Manning received help.
Lamo knew Manning via extensive e-mail conversation and later turned him in to authorities.
One civilian interviewed by the authorities told The New York Times that authorities may be concerned that there is more classified material in the Boston area.
The F.B.I., which can prosecute civilians, has been brought in to assist the military in the inquiry.