Manning, the 22-year-old army intelligence operative, suspected of leaking the documents to Assange, is being detained at a military installation in Virginia.
New evidence suggests that Manning initiated the theft himself, officials told The Wall Street Journal.
A further blow to investigators is that there is reportedly little to find a connection.
Earlier this year, the 39-year-old Australian computer hacker was catapulted into global spotlight when WikiLeaks released thousands of secret documents on the wars in Iraq and Afghanistan as well as a flood of State Department cables.
Since then, the US government has been trying to prosecute Assange for espionage but no specific charges against him have been framed.
The former Australian hacker has argued that all the actions taken by his organisation are protected under the First Amendment that protects free speech in America.
To make a case against Assange, the US lawyers would have to prove that he encouraged Manning to hand over the documents.
"Federal Bureau of Investigation agents and Justice Department lawyers continue to gather evidence for a possible conspiracy charge against Mr Assange, but that's a harder case to make," WSJ said, citing government officials.
"Army investigators now believe Pfc. Manning decided to steal the documents and give them to WikiLeaks on his own, out of his own malice toward the military or the government," the newspaper reported, citing a senior US official.
Meanwhile, Assange attended the second day of extradition proceedings in the UK.
The WikiLeaks founder is wanted by Sweden, which is investigating charges of sexual-assault allegations against him.
Last month, Assange warned that if a number of people from his organisation were "assassinated or imprisoned," a large number of "encrypted keys" containing large amounts of secret information would be released.
"There are backups distributed amongst many, many people, 100,000 people and all we need to do is give them an encrypted key and they will be able to continue on," he said in a television interview with CBS.
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