Maine’s two U.S. senators – both members of the Senate Intelligence Committee – stressed the importance Sunday of getting to the bottom of unsubstantiated assertions by President Trump on Twitter that President Obama had Trump Tower wiretapped during the presidential campaign last fall.

Republican Sen. Susan Collins said in a television interview that Trump ought to turn over any evidence he has of the alleged wiretapping to the Intelligence Committee in its investigation of Russian efforts to influence last fall’s presidential election.

Independent Sen. Angus King called the president’s tweets “unfortunate.”

“Apparently, he got his information from Breitbart News,” King said Sunday evening in Portland. “My thought was he has got this very large and very professional intelligence community. Maybe that’s a better source than Breitbart, particularly when you are going to make charges that are so incendiary and serious.”

“But I think now that the issue has been raised, we’ll get to the bottom of it,” King added.

In an appearance Sunday morning on the CBS News program “Face the Nation,” Collins also said it would be helpful if Trump stopped commenting on the investigation and “let us do our work.”

The senators’ comments followed a week of continuing questions about contacts between members of Trump’s campaign staff and Russian officials. Attorney General Jeff Sessions, a friend of Collins, recused himself Thursday from the FBI’s investigation into the alleged Russian interference. His recusal was triggered by news that he had misled the Senate Judiciary Committee during his confirmation hearings when he said under oath that he had not communicated with Russian officials as a surrogate for the Trump campaign. The Washington Post reported on Wednesday that Sessions had met with Russian Ambassador Sergey Kislyak twice during the campaign.

Collins said in the TV interview Sunday that the Senate Intelligence Committee’s investigation is in its early stages but she had seen no evidence to support Trump’s wiretapping claims.

King said he could see no reason why Trump’s allegations could not become part of Congress’ investigation into alleged contacts between the Trump campaign and Russia.

“We’ll see where it goes,” he said.

Asked by the host of “Face the Nation,” John Dickerson, whether Trump is obligated to provide any evidence backing up his accusations, Collins said she expects the president to turn over any evidence to the committee in order put the allegations to rest one way or another.

“My own theory is the Russians are determined to sow the seeds of discontent, and they were going to do that regardless of who was elected president. So it is really important we get to the bottom of this,” she said.

Asked in a telephone interview Sunday night if it was time to appoint a special prosecutor to investigate the Trump campaign’s alleged ties to Russia, King said, “I think it’s premature, but not out of the question.”

He said he would prefer that the Intelligence Committee complete its investigation first, and that appointment of a special prosecutor be considered only if there is evidence to suggest wrongdoing.

Collins said it is too soon to know if the committee will need to subpoena Trump’s income tax returns as part of its investigation.

“It will go where the evidence leads us and we will get all the information we need,” she said. “If that includes President Trump’s tax returns, I have confidence we will ask for them.”

King also said he was not prepared to support a subpoena of the tax returns. King said he would support such a move only if he felt the returns “were relevant and important.”

Collins was preceded on the “Face the Nation” program by Sen. Mark Warner of Virginia, the ranking Democrat on the Intelligence Committee, who said the panel will obtain all of the information it needs to get to the bottom of Russian interference in the 2016 presidential election.

In his tweets on Saturday, Trump offered no evidence to support his claims of wiretapping and an Obama spokesman called the accusations false.