Sen. Susan Collins on Wednesday left open the possibility of issuing a subpoena for President Donald Trump's tax records as lawmakers probe Russia's interference in last year's election.

The Maine Republican said the decision of whether to force the president to turn over his financial details would be made by Intelligence Committee Chairman Richard Burr R-N.C., and Vice Chairman Mark Warner, D-Va. as the committee attempts to determine the extent to which the Trump campaign was in contact with Russian officials.

"My hope is that we'll have a lot of voluntary cooperation," Collins told Maine Public Radio on Wednesday. "And keep in mind that in some ways, this is a counterintelligence operation – in many ways – and that's what our committee specializes in."

Collins also said former National Security Adviser Michael Flynn would be asked to testify before the committee.

Lawmakers began investigating Russia's meddling after the U.S. intelligence community concluded the Kremlin had encouraged the hacking of Democratic Party servers and released emails meant to embarrass their nominee, Hillary Clinton, and harm her election efforts.

A number of people within Trump's inner circle during the campaign have ties to Russia, including Flynn and Paul Manafort, who served as Trump's campaign chairman in the spring until questions emerged about his work for pro-Russian groups in Ukraine.

Those connections provoked suspicions that Trump – or people around him – were aware of Russia's efforts, and the questions intensified earlier this month after Flynn resigned following revelations he may have violated a law barring civilians from negotiating with foreign governments when he spoke with the Russian ambassador to the U.S. during the presidential transition.

Trump has repeatedly denied having any business interests or other ties to Russia and said he and his staff concluded that Flynn had done nothing wrong in contacting the Russian ambassador. The White House said the president asked for Flynn's resignation because he misled Vice President Mike Pence.

"There are law enforcement implications, and we want to make sure that we're doing it right," Collins said. "We're used to probing in-depth in this area."