Former Prince George’s county executive Jack B. Johnson admitted in federal court Tuesday that he took more than $400,000 in bribes, agreeing to a plea deal that showed he began shaking down developers almost from his first days in office.

Johnson’s plea in U.S. District Court in Greenbelt came on a day when federal prosecutors announced that the county’s former housing director and two developers had pleaded guilty in secret proceedings as part of a sweeping corruption investigation that remains active. Those cases show that the probe was in high gear as early as 2006, the same year Johnson sailed to a second term.

Johnson, 62, who had asserted his innocence and vowed to fight the charges as recently as March, pleaded guilty to extortion and witness- and evidence-tampering. There is no agreement on a specific punishment, but prosecutors said they will seek prison time at a sentencing hearing Sept. 15, and federal guidelines recommend a term of 11 to 13 years. He remains free until sentencing.

“This has been a very trying time for my family and I,” Johnson said after the hearing, declining to discuss specifics of the case. “I want to say to all the citizens of Prince George’s County, I’m very sorry for what happened. We all have sinned and fallen short of the glory of the Lord.”

U.S. Attorney Rod J. Rosenstein said that Johnson’s conviction marked “a milestone in the investigation” but that federal agents and prosecutors are still at work.

The case burst into the spotlight in November when Johnson and his wife, Leslie, 59, now a member of the County Council, were arrested at their brick colonial in Mitchellville. The couple were overheard on an FBI wiretap plotting to hide $79,600 in cash in Leslie Johnson’s bra and underwear and flush a $100,000 check from a developer down the toilet as federal agents knocked at the door, court papers say.

Both Johnsons were charged with evidence tampering, and Jack Johnson was later indicted on bribery and corruption charges. Prosecutors dropped the bribery charges as part of the plea. Rosenstein said Johnson’s deal would have no bearing on his wife’s case.

On Tuesday, Johnson hugged and greeted friends who had come to support him. In court, Johnson, who was the county’s top prosecutor before he served as county executive from December 2002 to 2010, told District Judge Peter J. Messitte that he was entering a guilty plea.

But as Assistant U.S. Attorney James Crowell prepared to describe Johnson’s wrongdoings, it appeared the deal might fall apart.

Messitte asked Johnson whether the admissions in the plea agreement were true.

“Generally, yes, sir,” Johnson said, prompting prosecutors to say the deal was off if Johnson did not fully admit guilt.

“I’m not trying to force this on you,” Messitte said. “You’re a former prosecutor, so you know.”

“I accept the document,” Johnson replied. Later in the hearing, Johnson said he wanted to talk about facts that could “explain” some of his actions. His attorney, Billy Martin, said that explanation would come at sentencing.

Leslie Johnson, who had been scheduled to plead guilty earlier this month, was not at her husband’s side Tuesday. She attended a council meeting at which she voted on a union contract issue. Just before the meeting ended, she left through a back door without acknowledging her husband’s guilt or her own case. Her plea hearing had been abruptly canceled.

County leaders said the case has bolstered the reputation of Prince George’s as a place that has long fostered cronyism and a pay-to-play culture.

“Clearly, this is a very sad day for Prince George’s County,” said Prince George’s council member William A. Campos (D-Hyattsville), who formerly worked for Johnson. “It is a black eye. We are talking about leadership.”

Johnson admitted that beginning in 2003, he and other public officials accepted cash, airline tickets, rounds of golf, campaign contributions and mortgage payments in exchange for influencing legislation, grant awards and even a restaurant health inspection.

Johnson steered millions in federal grant funding to a developer and arranged for jobs for friends who rewarded him with bribes.

As the end of his term in office approached, Johnson talked about his wife’s election to the County Council, telling one of the developers who bribed him that her position would be “real powerful.”

“You’ve got a good County Council,” Johnson said, according to the plea agreement. “And they’ll look out for you. I’ll have Leslie to, ah, take care of things.”

During two terms as county executive, Johnson was a popular figure who prided himself on transforming Prince George’s 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.

After Johnson’s hearing, federal officials also revealed details of the crimes committed by three men who have admitted guilt and are awaiting sentencing. One is a former county housing official who accepted bribes. The other two are developers who doled them out.

James Johnson, whom Jack Johnson appointed as the county housing director but is not related to the former executive, took tens of thousands of dollars in bribes from two developers. In return, he steered federal grant money to certain building projects and influenced building approval decisions.

Mirza Hussein Ali Baig, 67, a developer and Laurel doctor, paid $400,000 to $1 million in bribes to Jack Johnson and James Johnson, court papers said. Baig won at least three development deals in the county during a 19-month period while Johnson was county executive, according to a 2008 Washington Post investigation of Prince George’s development deals. Baig entered a secret plea in April.

Patrick Ricker, 52, a developer and president of Ricker Brothers, was the first of the three men to be convicted, entering a guilty plea in December 2009. Ricker and Baig cooperated in the probe, according to lawyers and court papers.

Prince George’s County Executive Rushern L. Baker III said in a statement that Johnson’s misdeeds are “no means reflective of the people of Prince George’s County or employees of its government.”

“Our obligation at this point is to restore this community’s trust,” Baker said. “The people of Prince George’s County are strong and resolute.”

Staff writer Hamil R. Harris contributed to this report.