Authorities had been unable to link the WikiLeaks founder to Bradley Manning, the army private jailed for passing confidential information to the whistleblowing website, NBC News said yesterday.
The network's chief Pentagon correspondent Jim Miklaszewski said sources inside the US military claimed they were struggling to find any evidence to prove Mr Assange and Pte Manning communicated with each other.
"The officials say that while investigators have determined that Manning had allegedly unlawfully downloaded tens of thousands of documents on to his own computer and passed them to an unauthorised person, there is apparently no evidence he passed the files directly to Assange, or had any direct contact with the WikiLeaks figure," Mr Miklaszewski said.
If the reports are true, authorities will be powerless to extradite Mr Assange to the US to face criminal charges relating to his website's leaking of classified documents.
The recent release of a massive cache of US cables angered and embarrassed the US, leading the Obama Administration to label Assange a "hi-tech terrorist".
News reports late last year revealed the White House was exploring options of criminally prosecuting Mr Assange under the Espionage Act.
US authorities were reportedly trying to build a criminal conspiracy case against Mr Assange, to prove he helped Pte Manning when the soldier allegedly copied more than 250,000 classified US government cables on to a CD and smuggled the data to WikiLeaks.
Australian-born Mr Assange, 38, is on bail in England while he waits to face a London court on February 7 for extradition to Sweden on sex offences that allegedly occurred last year.
Mr Assange has claimed he had never heard of Pte Manning.
But the 23-year-old soldier, in solitary confinement since July, allegedly communicated with Mr Assange and WikiLeaks on Twitter, according to reports last year.