Nearly 3,500 people have been exonerated of crimes in the U.S. since 1989, and $4 billion has been paid in compensation.

Of the nearly 3,500 people exonerated of serious crimes in the U.S. since 1989, more than half had their cases marred by alleged misconduct by police or prosecutors, according to a national database.

But experts say it’s rare for anyone to be held accountable for the harm — for the coerced confessions, hidden evidence, false testimony and other dubious work that contributes to flawed convictions.

The pending perjury trial of three retired Philadelphia police detectives could prove an exception, if they themselves are not cleared by alleged mistakes by District Attorney Larry Krasner’s office. Former detectives Martin Devlin, Manuel Santiago and Frank Jastrzembski have asked a judge to dismiss the case. The judge plans to rule by April.

An unusual confluence of factors allowed Krasner to charge the three in the case of exoneree Anthony Wright, who in 1993 was convicted of the 1991 rape and murder of an elderly widow. Chiefly, the detectives had testified at his retrial in 2016, reopening a five-year window to file perjury charges.