Longtime Trump confidant Roger Stone sent a letter to U.S. Attorney John Durham this week seeking answers about whether he was a subject of surveillance by the U.S. government.

“We have strong reason to believe that Mr. Stone was among three advisors to candidate Trump who was under surveillance by the FBI during the 2016 presidential campaign,” Stone’s lawyer Paul Jensen wrote in the letter obtained by The Hill. Jensen also said Stone’s Fourth Amendment rights may have been violated.