Harvard law professor Alan Dershowitz said Thursday that Michael Cohen's plea agreement with Robert Mueller indicates the special counsel is broadening his investigation beyond criminal matters.

"This new development with Cohen suggests that they're going to go after political issues, not just criminal issues," Dershowitz told Hill.TV's Buck Sexton in an interview airing Friday on "Rising."

Court documents on Thursday revealed that Cohen admitted to lying about President Trump's property plans in Moscow, as well as his plans to travel to Russia and his contacts with Russian officials linked to the project and how long the property plans were discussed within the Trump Organization.

"Let's just assume for hypothetical purposes that it's true -- President Trump was trying to build a tower in Moscow in the early stages of the campaign. That would not be a crime. What is Mueller doing investigating that?" Dershowitz asked. "What's the federal crime in wanting to build a tower? It might be a federal crime by Cohen to lie about it, but how does this involve the possibility the president may have committed a crime?"

Mueller is probing Russian election interference in the 2016 presidential election and possible collusion between Moscow and the Trump campaign.

The president continued to rail against the Russia investigation on Thursday, which he frequently refers to as a "witch hunt."

"When will this illegal Joseph McCarthy style Witch Hunt, one that has shattered so many innocent lives, ever end-or will it just go on forever?" he tweeted early Thursday. "After wasting more than $40,000,000 (is that possible?), it has proven only one thing-there was NO Collusion with Russia. So Ridiculous!"

Mueller's investigation has led to four guilty pleas from former Trump associates, and he has obtained indictments against more than 20 Russian nationals.

- Julia Manchester