Jack Johnson, center, arrived at the courthouse Tuesday with attorney Billy Martin, left, and pastor Jonathon Weaver. The former Prince George’s Co. executive pleaded guilty Tuesday in a corruption probe. (Sarah L. Voisin/The Washington Post)

Johnson, 62, who was at the helm of one of the nation’s most affluent majority-black counties for eight years, pleaded guilty to two felony counts, extortion and conspiracy, and witness and evidence tampering during an afternoon hearing in U.S. District Court in Greenbelt.

“This has been a very trying time for my family and I,” Johnson said outside the courthouse after the hearing. “I want to say to all citizens of the county, I’m very sorry for what happened. We all have sinned and fallen short of the glory of the Lord.”

The plea deal does not include a specific sentencing recommendation, but federal guidelines indicate that Johnson could spend 11 to 13 years in prison.

Federal prosecutors on Tuesday also revealed that one former public official and two developers already have pleaded guilty in secret proceedings in connection with the investigation.

Patrick Ricker, 52, a developer and president of Ricker Brothers Inc., pleaded guilty in December 2009 to charges including conspiracy to commit fraud and tax evasion. Ricker recruited “straw donors” to conceal campaign contributions to state and federal candidates, authorities said.

In January 2011, James Johnson, 66, the former director of the Prince George’s Department of Housing and Community Development, pleaded guilty to conspiracy to commit extortion. Authorities said he recieved bribes from developers.

And in April, Mirza H. Baig, 67, a physician and developer with ties to Jack Johnson, pleaded guilty to to conspiracy to commit extortion in connection with bribes he paid to Jack Johnson and James Johnson.

Jack Johnson’s admission marked a grim coda to his long career of public service in Prince George’s County. The democrat served eight years as county executive, leaving in December after term limit rules barred him from a third bid. Before that, he was the county’s top prosecutor for two terms.

The plea comes six months after Johnson and his wife, Leslie, a member of the County Council, were arrested at their brick colonial in Mitchellville in a case that attracted international attention. The couple was overheard on an FBI wiretap plotting to hide $79,600 in cash in Leslie Johnson’s bra and flush a $100,000 check from a developer down the toilet as federal agents knocked at the door, federal authorities said.

The case against Johnson comes as part of a sweeping, six-year corruption probe in Prince George’s County that reached deep into the ranks of power players in the tight-knit government and business communities.

The Johnsons are among nine people publicly charged in the investigation. Two county police officers are charged with taking part in a black-market cigarette smuggling scheme, and a third officer is charged with cocaine trafficking.

Authorities ultimately charged Johnson with bribery, alleging that he had been shaking down developers and business owners during nearly all his time as county executive. They said in court papers that he accepted more than $200,000 in bribes from a developer and demanded donations for his wife’s council campaign.

But even as the FBI released transcripts of a recorded conversation in which Johnson urged his wife to flush the $100,000 check, the former county executive proclaimed his innocence.

The day of his arrest, Johnson stood outside the Greenbelt courthouse and declared he was “absolutely convinced I’ll be vindicated.” At a March hearing, Johnson entered a not guilty plea and again said he would fight the charges. “My reputation is at stake,” he said.

During two terms as county executive, Johnson was a popular figure who prided himself on transforming Prince George’s County into “Gorgeous Prince George’s.” During his tenure, the county’s Wall Street Bond rating was raised, and he took credit for improving emergency services and attracting businesses.

But he was also criticized for doling out government contracts to allies not qualified for the work, and for excessive charges on the county credit card.

In 2006, federal authorities launched an investigation that they say revealed that that real estate developers were “regularly providing things of value” to public officials in exchange for official favors.

Over the following years, FBI agents listened in on wiretaps, worked with informants and searched county offices, businesses and private homes, court papers say. Authorities began tapping Jack Johnson’s cell phone in January 2010.

Leslie Johnson, 59, who was elected to the council Nov. 2., has maintained her seat despite being stripped of some responsibility by the full council. She is charged with conspiracy to commit witness and evidence tampering, a felony. If she is convicted of a felony, she would have to step down from the council, officials have said.

Jack Johnson is the only person who has been publicly charged with bribery, but other public officials and busi­ness­peo­ple are described as co-conspirators in court papers.