The House Intelligence Committee's investigation is just one of several congressional probes into Russian Federation, along with one by the Federal Bureau of Investigation.
The memorandums - the contents of which have been leaked to the media and have not been denied by Comey - reportedly document the president's efforts to get the FBI to ease the investigation's focus on Flynn. The committee is now investigating claims that the Trump campaign colluded with Russian Federation to influence the 2016 presidential elections in its favor.
Trump has repeatedly dismissed allegations that his campaign collaborated with Russian Federation ahead of the presidential election.
At next week's hearing, Mr Comey is expected to be asked about conversations in which Mr Trump is reported to have pressured him to drop an investigation into former national security adviser Michael Flynn, whose ties to Russian Federation are under scrutiny.
Trump has strongly denied that either he or his campaign team in any way cooperated with Russian officials during last year's election. However, whether or not the FBI was investigating President Donald Trump or not, it was improper and unlawful for Trump to ask the director this question. The reports contained no evidence that any Americans were targets of U.S. eavesdropping operations, they said.
The Senate panel hearing is expected to quiz Comey on memorandums he allegedly wrote on three conversations he had with Trump in January and February.
Mr Trump on Thursday renewed his allegation, without citing evidence, that his campaign communications were monitored, saying in a tweet: "The big story is the "unmasking and surveillance" of people that took place during the Obama administration". Trump asked for his resignation less than a month after the administration took office amid a scandal prompted by Flynn's misleading of senior officials, including Vice President Mike Pence, about his contacts with Russia's USA envoy.
It is reasonable to conclude, the source added, that Comey did not think any of Trump's individual actions constituted obstruction of justice.
The committee's ranking Democrat, Rep. Adam Schiff of California, said Nunes approved the unmasking subpoenas without consulting with Democrats on the committee. The GOP staffer, who was not authorized to discuss the issue publicly and spoke only on condition of anonymity, said the chairman has the authority to sign off on subpoenas and that all the committee rules were followed.
The Guardian newspaper said on Thursday said that Nigel Farage, a Trump supporter and a leading Brexit campaigner, is a "person of interest" in the Federal Bureau of Investigation probe but has not been accused of wrongdoing.
Until he was sacked on May 9, Comey was leading the FBI's investigation into Russian meddling, which had started back in July.
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