(CNN) Sen. Susan Collins, a member of the Senate Select Committee on Intelligence, argued that President Donald Trump was not immune to an investigation into his financial dealings by Justice Department special counsel Robert Mueller.

"The President can't set red lines for Bob Mueller," the Republican from Maine told CNN's Dana Bash in an exclusive joint interview that also featured Sen. Lisa Murkowski, R-Alaska.

"Well said," Murkowski added.

CNN reported Thursday that Mueller and his team are now looking into Trump and his associates' financial ties to Russia as they investigate the 2016 presidential election and whether there was any collusion between the Trump campaign and Russian officials.

Asked in a New York Times interview last month if he felt that a probe into his finances would cross a red line or be a breach by Mueller, Trump said "yes." Trump insisted he doesn't make money from Russia, but said that someone from Russia may have purchased real estate from him.

"It's possible there's a condo or something, so, you know, I sell a lot of condo units, and somebody from Russia buys a condo, who knows? I don't make money from Russia," he said. "They said I own buildings in Russia. I don't. They said I made money from Russia. I don't. It's not my thing."

Collins, who's been on the committee that has spearheaded a Senate investigation into the election that's separate from that of Mueller, said the special counsel "should follow the leads wherever they may be."

"I do not think his investigation should be constrained beyond the mandate that he was given," she said.

CNN also reported Mueller has issued grand jury subpoenas related to Donald Trump Jr.'s 2016 meeting with a Russian lawyer at Trump Tower. The grand jury activity was first reported by the Wall Street Journal and Reuters.

Collins said that such a step signals "the investigation is progressing to a new stage."

"I think that's good," she continued. "The sooner that Bob Mueller and his team are able to get the facts out about Russian involvement and answer the question of whether or not there is any criminal wrongdoing or collusion between the Russians and members President Trump's campaign team, the better off we all are."

For her part, Murkowski said that if Mueller "feels that it is appropriate to impanel a grand jury, I think we need to listen to where he is taking this."

Trump's lawyer, Jay Sekulow, said Thursday on Fox News that they have "no reason to believe that the President is under investigation here."

"To look at a real estate deal from 10 years ago would be way outside the scope of the mandate and we would certainly object to that," Sekulow said when asked about the red line.