U.S. Sen. Susan Collins said on Wednesday she's open to using a subpoena to investigate President Donald Trump's tax returns for potential connections to Russia.

Collins, a Republican who has served as a U.S. senator from Maine since 1997, sits on the Senate Intelligence Committee, which is investigating Russian interference in the 2016 election. She appeared on Maine Public radio to talk about issues including the investigation.

Collins was asked if the committee would subpoena Trump, who's also a Republican. She said she hopes for "voluntary cooperation" but is open to using a subpoena if necessary.

"This is a counter-intelligence operation in many ways. That's what our committee specializes in," she said during the radio appearance. "We are used to probing in depth in this area."

Trump's refusal to disclose his tax returns is a break with presidential tradition. He has said he would be happy to release them after the completion of an Internal Revenue Service audit.

Using a subpoena to get access to the tax returns would be a more aggressive move than members of Congress have taken on the subject so far. House and Senate leaders have thus far shown no interest in taking such a step.

Last week, House Republicans blocked an attempt by Democrats to use an obscure law to obtain the tax returns from the IRS. Democrats on the House Ways and Means Committee tried to frame the issue as a matter of national security, questioning whether Trump has any investments in Russia.

Collins also said during the radio appearance that she and other intelligence committee members will call for former national security adviser Michael Flynn to testify before the committee. Flynn resigned following reports that he had misled officials about his contacts with Russia.

Collins said the committee is in the midst of a "broad investigation" about Russian interference and it's too early to speculate about the results.

She touted the "bipartisan" nature of the committee's probe and pledged: "We will get to the bottom of this."

Meanwhile, Collins has been getting heat in her home state for declining to hold a town hall. Republican members of Congress around the country have had their town halls interrupted by protesters in recent weeks.