PORTLAND, Maine — Republican Maine U.S. Sen. Susan Collins says she's open to using a subpoena to investigate Republican President Donald Trump's tax returns for potential connections to Russia.

Collins sits on the Senate Intelligence Committee, which is investigating Russian interference in the 2016 election. She appeared on Maine Public radio Wednesday to talk about issues including the investigation.

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

Trump has refused to disclose his tax returns, saying he's under IRS audit.

Collins also says she'll call for former national security adviser Michael Flynn to testify before the intelligence committee. She says the committee is in the midst of a "broad investigation" about Russian interference and it's too early to speculate about the results.