Prince George's County Executive Jack B. Johnson outside the federal courthouse in Greenbelt, Md., in 2010. (Marvin Joseph/The Washington Post)

Former Prince George’s County executive Jack B. Johnson, who pleaded guilty in a broad corruption scheme and is serving 87 months in prison, is asking a Maryland judge to set aside his conviction and sentence.

In a motion filed in U.S. District Court of Maryland, Johnson claims he has “newly discovered evidence” that “uncovers probable law enforcement misconduct” in connection with the federal investigation into him that ended with his 2011 plea on extortion and witness and evidence tampering.

The latest motion claims that Johnson and his family received several “offensive and hate-filled” notes “containing implied death threats,” before Johnson reported to prison in 2012. He suspects that a member of the Prince George’s County Police Department sent them, though the motion acknowledged that there was no solid evidence to prove it.

At least one note was sealed in a Prince George’s County government envelope and mailed to Johnson’s home in Mitchellville, according to court records. That envelope contained Monopoly money with handwritten notes on the back urging Johnson to “Rot in Jail.” Another was sent to the College Park home of his eldest son, Jack Johnson Jr. One of the notes referenced two county police officers whom Johnson prosecuted while he was state’s attorney from 1994 to 2002.

“I have always suspected that law enforcement, involved in prosecuting the case against me, sent these hate-filled and threatening message to me and my family,” Johnson said in a sworn declaration accompanying his motion.

Terry Eaton, Johnson’s attorney, declined comment other than to say: “We look forward to litigating the issues we raised in court.”

Johnson hired private investigators and a forensic lab to help determine who was involved in the alleged intimidation, including possibly federal agents, according to records. The lab found DNA trace evidence on the back of the envelope mailed on county government stationery and said it matched DNA from a second envelope.

Johnson, 67, said he would not have pleaded guilty had he “known that it was possible to recover DNA evidence” from the envelopes.

Maryland U.S. Attorney Rod J. Rosenstein, whose office prosecuted Johnson and his wife, said Johnson’s motion is without merit.

“Mr. Johnson admitted under oath that he was guilty of serious crimes,” Rosenstein said in a statement on Monday. “His motion does not include anything that casts doubt on the propriety of his conviction and sentence.”

Johnson, who served as county executive from 2002 until December 2010, masterminded a corruption conspiracy and received more than $1.6 million in bribes, and implicated his wife, Leslie Johnson, and several developers, county officials and businessmen.

Johnson and his wife were arrested at their Mitchellville home by the FBI in November 2010 as part of a sting operation. They were overheard on a wiretap scheming to flush a $100,000 check that he received as a bribe down the toilet, and stash $79,600 in cash in her underwear. Jack Johnson also was videotaped taking cash bribes from a longtime associate and developer.

A grand jury returned an eight-count indictment against the Johnsons in February 2011.

Jack Johnson, who could have received a maximum of 14 years in prison, agreed to a plea deal and was sentenced to 87 months. The judge also fined him $100,000 and ordered him to undergo alcohol treatment and forfeit $78,000 and his antique Mercedes-Benz.

Johnson’s sentence was among the longest in Maryland history for a politician in a corruption case.

He is in custody at the Federal Correctional Institution in Cumberland, Md., and is scheduled to be released in June 2017, according to the Federal Bureau of Prisons.

Leslie Johnson, who was elected to the County Council 10 days before her arrest, pleaded guilty to one felony count of conspiracy to commit witness and evidence tampering. She received one year and one day in prison, and was released in 2013.