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Swedish tabloid Expressen reported on Thursday that a police officer involved in the initial phase of the probe had personal and political links to one of the two women accusing Assange of sexual misconduct.
Expressen also said the officer, Irmeli Krans, described Assange as a "bubble ready to burst" on her personal Facebook page.
"If this information is correct, then one should carefully consider whether the nature of the investigation is such that he can be assured a fair trial," Assange's lawyer Bjoern Hurtig told The Associated Press.
Assange denies the allegations, which stem from a visit to Sweden in August.
He is currently fighting extradition from Britain, where his lawyers have argued, unsuccessfully so far, that he won't be granted a fair trial if handed over to Sweden.
They also said he risks being handed over to the US, which is investigating whether WikiLeaks violated US laws by releasing tens of thousands of secret government documents.
Expressen said Krans knew one of Assange's accusers from their involvement in Sweden's left-wing Social Democratic Party. Krans didn't answer calls and an email seeking comment on Thursday.
Stockholm police spokesman Ulf Goranzon rejected any conflict of interest in the investigation. He said Krans didn't interview her supposed friend, but the other woman, and wasn't involved in the investigation after that.
As for the Facebook comments, Goranzon said "it is important for police officers to contemplate what they say and how they say it on social media."