Federal investigators have concluded their probe of Robert “Bud” McFarlane, whom they once suspected of having an inappropriate relationship with the government of Sudan, and will not file any criminal charges against the Reagan administration national security adviser, his attorney said Wednesday.

Barry Wm. Levine said prosecutors had told him that “the investigation they did was thorough and it’s closed.” He declined to discuss prosecutors’ reasons for terminating the case, but said McFarlane was “totally innocent of any allegation,” and the prosecutors’ decision not to charge him demonstrated that.

“He’s exonerated,” Levine said. “He’s vindicated.”

A spokeswoman for the U.S. Attorney’s Office for the Eastern District of Virginia declined to comment.

The FBI began investigating McFarlane, who is known for his role in the Iran-contra scandal, after a 2009 Washington Post article outlined his ­involvement with the strife-torn African nation of Sudan, which has long sought to ease U.S. economic sanctions and to be removed from the State Department’s list of state sponsors of terrorism.

The investigation, which was being conducted in secret, burst into public view earlier this year when federal agents searched McFarlane’s ninth-floor Watergate condominium. They alleged in a publicly available search warrant that McFarlane was “entering into an agreement with the government of Sudan to lobby the U.S. government officials on behalf of Sudan.”

U.S. law makes it a crime to work as an agent of a foreign government without proper disclosure and prohibits business with Sudan because of its history of human rights violations in its decades-long civil war.

Before searching McFarlane’s condominium for business records, the FBI had in 2010 searched his business e-mail account and went through the trash of the former offices of his consulting firm in Arlington, according to court documents.

Levine said the search warrant should not have been made public and that it reflected the “misguided” perception of federal investigators rather than any formal accusation of criminal wrongdoing. A search warrant is not a formal charge, and federal investigators often conduct searches in cases where they ultimately conclude no wrongdoing has occurred.

Levine said McFarlane was doing business with Qatar, not Sudan, and he was concerned especially with the people of Darfur who were “suffering at the hands of the Sudanese.” He said he believed McFarlane’s efforts in that region were over, and McFarlane was now working on projects related to global energy and improving life in Southeast Washington.