There was also no immediate confirmation from the Senate committee.
"The U.S. Senate Select Committee request confirms their interest in hearing from Mr Assange", lawyer Jennifer Robinson said in a statement.
Assange has remained sequestered, for the time being, in the Ecuadorian embassy in London, where he has been given diplomatic asylum after his controversial website Wikileaks published thousands of confidential defense and diplomatic documents in 2010.
The letter, which could be a fabrication, is marked 1 August and was addressed to the Ecuadorian embassy, where Assange has been a resident since 2012.
"The committee requests that you make yourself available for a closed interview with bipartisan committee staff at a mutually agreeable time and location", the letter reads.
Moreno said he is negotiating with the British government over the release of Wikileaks founder Julian Assange, who has been holed up in the embassy for six years.
Sweden dropped its investigation into Assange over allegations of sexual offenses in May 2017.
Julian Assange, who has been holed up in the Knightsbridge residence since 2012, over fears he will be extradited to the United States to face prosecution over WikiLeaks' publication of leaked U.S. military and diplomatic documents.
Ecuador and Britain are now in talks over his fate.
Twelve Russian military intelligence officers hacked into the Hillary Clinton presidential campaign and the Democratic Party and released tens of thousands of private communications in a sweeping conspiracy by the Kremlin to meddle in the 2016 USA election, according to an indictment announced days before President Donald Trump's summit with Russian President Vladimir Putin in July. In July, special counsel Robert Mueller indicted 12 Russian military intelligence officers allegedly behind the "fictitious persona" Guccifer 2.o.
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