Deputy Attorney General Rod Rosenstein’s memo recommending the firing of former FBI Director James Comey could be part of a criminal investigation into the president’s relationship with Russia, according to one Democratic senator.

“There were a lot of questions yesterday about the Rosenstein memo,” Sen. Sheldon Whitehouse, D-R.I., said on MSNBC Friday, according to The Washington Times. 

Rosenstein wrote a memo to President Donald Trump advising him to fire Comey, which the White House cited as reason for his dismissal, though Trump himself said later that he had already made up his mind to fire the director.

The deputy attorney general said of the memo Friday, “I wrote it. I believe it. I stand by it,” according to Fox News. 

“Rod was very cautions about answering any of those questions because he said it is possible for the very creation of that Rosenstein memo to be a part of [special prosecutor Robert Mueller’s] investigation. And until it’s clear it’s not going to be, he doesn’t want to start muddying up the waters for the Mueller investigation.”

Whitehouse added that the different investigations into Trump need a “system in place” to keep from overlapping.

“There’s going to have to be deconfliction between our legitimate oversight and legislative investigative efforts and his criminal and investigative efforts. And we’re going to need to get a system in place so that that deconfliction can take place through a responsible air traffic control,” he said, adding that he didn’t know if Trump would be investigated himself.

“I don’t know that. And I think Bob Mueller will have to make that call. I think it’s clear the investigation leads into that circle.”