The State House in Annapolis (Mark Gail/The Washington Post)

A longtime Maryland law­maker was charged in federal court Friday, accused of using his official position to help a phony real estate development in exchange for thousands of dollars in bribes.

Investigators say state Sen. Nathaniel T. Oaks (D-Baltimore), a member of the powerful Senate Finance Committee, misused his State House letterhead and introduced legislation for the fake real estate project of a businessman who gave the lawmaker more than $15,000 in cash last year. The businessman was actually working for the FBI.

Oaks, who served more than two decades in the House of Delegates before becoming a senator in February, was scheduled to appear before a judge Friday afternoon in U.S. District Court in Baltimore. He faces a maximum penalty of 20 years in prison, according to the office of the Maryland U.S. attorney.

The accusations filed Friday make Oaks the third current or former Maryland lawmaker charged criminally since January, the start of the 90-day legislative session.

Del. Michael L. Vaughn (D-Prince George’s) resigned minutes before the start of the session in January and was later charged for alleged involvement in a wide-ranging bribery scheme. Former Del. William A. Campos (D-Prince George’s) was also accused of accepting bribes in the federal investigation.

Senate President Thomas V. Mike Miller, Jr. (D-Calvert) said he was “very disappointed” and “shocked” and had urged Oaks to turn himself in when they met privately Friday morning.

Oaks, 70, was appointed to fill a Senate seat vacated by Lisa Gladden, a Democrat who retired in January. She has multiple sclerosis.

Oaks’s attorney, Stuart O. Simms, declined to comment on the allegations.

According to the criminal complaint charging Oaks with “honest services fraud,” the state legislator attracted the attention of federal authorities through a cooperator who was the subject of a separate FBI investigation.

In a meeting at a Ruth’s Chris Steak House in a Baltimore suburb in 2015, Oaks was introduced to an FBI source who portrayed himself as a businessman looking to obtain contracts in Baltimore through a minority-owned company, court records show.

In the months that followed, Oaks had an assistant type up two letters on his House of Delegates stationery at the businessman’s request in support of what Oaks thought was a Department of Housing and Urban Development project, according to the court filing. Investigators assert Oaks knew the letters contained false statements about his involvement and knowledge of the project.

Oaks later introduced state legislation at the businessman’s request to fund up to $250,000 for the project, according to the court filing.

Throughout the investigation, Oaks expressed concern about his calls being monitored by law enforcement, court records state. He and the FBI source agreed to use the code word “lollipop” to represent $1,000, the records say. Last June, investigators say, the source bought Oaks a prepaid phone to use for their conversations.

Oaks has been in legal trouble before. In 1988, he lost his House seat after being convicted of stealing thousands of dollars from his reelection fund. He received a five-year suspended sentence.

In 1994, he made a comeback, rejoining the House, He served there until he was appointed in February to the Senate seat.

Gov. Larry Hogan (R) has pushed this legislative session for stronger ethics laws following the federal investigation that led to the arrests of Vaughn and Campos. The measure, which received final passage in the Senate on Friday, would increase financial disclosure requirements and expand the definition of what constitutes a conflict of interest.

“People who hold public office have to be held to a higher standard than everybody else,” Miller said of the ethics law. “They are the ones that are making the laws, and they can’t be the lawbreakers.”

A recent Washington Post-University of Maryland poll found that nearly a third of Maryland residents see corruption as a major problem in state government.