The US Senate Intelligence Committee has issued a subpoena demanding documents related to Russian Federation from President Donald Trump's former national security adviser Michael Flynn, ramping up its investigation of Moscow's alleged meddling in the 2016 USA election.
The Senate Intelligence Committee said a subpoena had been issued after Flynn rejected its request on April 28 to submit documents relevant to the investigation.
In a joint statement released on Wednesday, Burr and Warner said the subpoena covers documents related to the committee's investigation into Russia's alleged election meddling.
Flynn, who was named by Trump as his national security adviser after the election, emerged at the center of the controversy when it was revealed he secretly discussed United States sanctions with the Russian ambassador to the USA in December.
On Monday, the former acting attorney general, Sally Yates, told lawmakers that she had warned the president's top legal adviser on January 26 that Flynn's deception on those calls left him open to blackmail.
Russian Federation and the Trump administration deny any part in colluding on the USA election.
Burr is one of several prominent Republicans who have expressed strong doubts about the timing of Trump's action. He said "the Russian collusion scandal is without any evidence to this day".More news: Ex-congresswoman found guilty of using funds from fake charity
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Burr has said he was troubled by the Comey firing but would continue to press ahead with his investigation wherever it leads.
An attorney for former National Security Adviser Michael Flynn has declined to comment on the issuance of a subpoena from the Senate intelligence committee.
Mr Comey's sacking prompted a storm of criticism from Democrats, who accused the president of seeking to stall the probe.
Washington D.C. [United States], May 12: United States President Donald Trump on Friday threatened sacked Federal Bureau of Investigation chief James Comey not to "leak" details of their conversations to the media.
The Flynn subpoena also comes one day before the committee is scheduled to hold an annual hearing with the heads of the various government intelligence agencies to discuss worldwide threats.
Comey was astonished by Trump's wiretapping claims and told aides, according to The New York Times, it was "outside the realm of normal" and perhaps "crazy".
The official White House readout of Trump's meeting didn't mention that Russian ambassador Sergey Kislyak was there.