Updated

The House Committee on Ethics voted unanimously Monday to formally investigate Florida GOP Rep. Trey Radel’s cocaine conviction.

Radel pleaded guilty Nov. 20 to a misdemeanor charge of cocaine possession and was sentenced to one year of probation. He is on leave of absence from Congress until after the first of the year.

Radel's lawyer ,David Schertler, said after his client admitted to buying 3.5 grams of cocaine from an undercover police officer in October that Radel had already entered outpatient treatment in Washington and would seek treatment in Florida.

The last Capitol Hill lawmaker arrested and convicted on drug charges was Rep. Frederick Richmond, D-N.Y., in 1982. He was convicted on a marijuana charge. He also pleaded guilty to felony charges of tax evasion.

The ethics’ committee will now create an investigative subcommittee led by Pennsylvania GOP Rep. Charlie Dent. New York Rep. Yvette Clarke will serve as the top Democrat. The group also will include Reps. Marsha Blackburn, R-Tenn., and Janice Hahn, D-Calif.

The committee says that it will determine whether Radel "violated the Code of Conduct or any law, rule, regulation or other applicable standard of conduct in the performance of his duties or the discharge of his responsibilities, with respect to conduct forming the basis for criminal charges of possession of cocaine."

A source told Fox News the Republican-led House "doesn't want to kick someone when they're down" but that sometime next year "the pink slip may come."

The pressure of an ethics probe, coupled with the conviction, could make it hard for Radel to continue to serve in Congress, which could pressure him to resign before the inquiry is complete.

As part of a plea agreement, Radel acknowledged he agreed to buy the cocaine for $250 in a Washington, D.C., neighborhood on Oct. 29. After the undercover officer gave Radel the drugs, federal agents confronted him, court documents show. Radel agreed to talk with the agents and invited them to his apartment, where he also retrieved a vial of cocaine he had in the home, the documents state.

A DEA official who spoke on the condition of anonymity because he was not authorized to release details of the case in his own name said Radel was identified to authorities as a cocaine buyer by his suspected dealer. The dealer had been arrested previously as part of a separate drug investigation led by a federal task force.

The Associated Press contributed to this report.