Maryland’s highest court on Monday rejected Prince George’s County’s request to throw out a civil judgment of $5 million for a man who was jailed for more than eight months after he was wrongly charged with killing his wife — a decision that will cost the county at least another $2.5 million.

Under Maryland law, the plaintiff in the case, Keith Longtin, began accruing 10 percent interest from the day a civil jury’s verdict was entered into the record in 2006.

The civil jury found that county police had violated Longtin’s constitutional rights and awarded him $6.4 million on Aug. 31, 2006. A Circuit Court judge later reduced that award to about $5 million.

But attorneys for the county asked the Court of Special Appeals to throw out the verdict, arguing that Longtin had failed to meet a filing deadline. He was in jail for the slaying he did not commit when the deadline passed.

In January 2010, a three-judge panel of the Court of Special Appeals rejected the county’s appeal.

Rather than pay Longtin and his attorneys, county attorneys again appealed, to the state Court of Appeals. Interest continued to build.

“It works like a credit card,” said Cary J. Hansel, one of Longtin’s attorneys. “The interest gets compounded.”

By the time the Court of Appeals upheld the civil award Monday, an additional $2.5 million in interest had accumulated, Hansel said. The county now owes Longtin and his attorneys $7.5 million, and interest mounts each day the award is not paid, Hansel said.

“The irony is that the county turned down repeated offers for much lower amounts,” said Timothy F. Maloney, another member of Longtin’s legal team.

County Attorney Stephanie P. Anderson declined to comment through the person who answered the phone at her office Monday.

Monday’s 51-page decision has broader implications for civil rights law in Maryland, Hansel said.

The Court of Appeals established the right for plaintiffs to bring “pattern or practice” claims against local governments. For example, Hansel said, a plaintiff who alleged he had been unlawfully beaten by a local police officer will now be able to show the jury evidence that the officer — or the officer’s department — has a history of such behavior.

“It’s absolutely historic,” Hansel said of the appellate ruling. “Previously, Maryland courts were limited to reviewing each case piecemeal. Prior misconduct, even of the same nature, was often hidden from the jury, no matter how egregious or extensive. Now, where there is a history of abuse, juries will be made aware of prior civil rights violations.”

In cases involving allegations of police misconduct, the ruling means plaintiffs’ attorneys will have access to the disciplinary records of officers, something that police departments have resisted disclosing, Hansel said.

Longtin spent more than eight months in jail in 1999 and 2000 on charges that he killed his estranged wife, Donna Zinetta, 36. Zinetta’s body was found in woods near her Laurel apartment in October 1999. She had been stabbed and slashed in the face, neck, and chest.

County homicide detectives interrogated Longtin for more than 28 hours without sleep and ignored his requests for a lawyer. Detectives said Longtin confessed, but Longtin denied that. Longtin’s case was one of four chronicled in a Washington Post series on false confessions obtained by Prince George’s police.

In January 2000, DNA results cleared Longtin, but police did not notify prosecutors. Charges against Longtin were dropped five months later.

In 2001, another man was convicted of murdering Zinetta and sentenced to two life terms.