American investigators probing the leak of classified documents related to Afghan war offered cash to a computer expert to "infiltrate" whistle-blower website WikiLeaks to prevent further disclosures, a media report said today.

The investigators interviewed the Boston area acquaintance of Bradley E Manning, accused of giving WikiLeaks State Department cables and a video of a helicopter attack in which unarmed civilians were killed in Baghdad, to contain further leaks, The Washington Post reported.

The man, a computer expert who met Manning in January, said he told the investigators in mid-June that he knew of no such documents.

Officials have said they are investigating whether Manning leaked the Afghanistan documents made public last week, a disclosure that prompted condemnation from the Obama administration.

The computer expert said the Army offered him cash to, in his word, "infiltrate" WikiLeaks.

"I turned them down," he said. "I don't want anything to do with this cloak-and-dagger stuff."

Army Criminal Investigation Division spokesman Chris Grey declined to comment on the claim.

"We've got an ongoing investigation," he said. "We don't discuss our techniques and tactics."

Another Manning acquaintance, who was questioned, said investigators "assumed that he was the one who did it and were trying to understand why, what was going on with him psychologically, to either make it so nobody gets to this point in the future or spot people who've gotten to this point and make sure they didn't do any damage."

This acquaintance, also a computer expert who spoke on the condition of anonymity, is affiliated with the prestigious Massachusetts Institute of Technology.

He said he was interviewed twice in June in Cambridge, Mass, shortly after Manning was detained. Manning was charged in July.

Manning, who lived in Potomac and was stationed at Fort Drum, N.Y., before shipping out to Baghdad last year, had hoped he would serve his time and then use the G.I. Bill to go to college. His military attorney has declined to comment.

"He was definitely interested in making a positive impact on the world," said Danny Clark, a friend of Manning's who runs a small tech firm in Cambridge and has declined to be interviewed by military investigators.

Manning has been transferred from Kuwait, where he had been detained, to Quantico. He was charged in military court in July and will have a preliminary hearing to determine if he should face a court-martial.

At the time of his arrest, Manning was an intelligence analyst at a relatively small base in Iraq.