A majority of the city’s murders since 2015 were committed by suspects who should have been in prison.
 

Baltimore isn’t the country’s murder capital—that distinction belongs to St. Louis—but it’s close. Charm City has recorded more than 2,500 homicides since 2015. Many of these killings could have been prevented, my analysis of court records and police data suggests, if the justice system had worked as intended.
 

Look no further than the case of Deonte Walker, convicted of the January 2020 murder of Justin Antonio Johnson. Less than three years before the killing, Mr. Walker was charged with at least 10 counts, and possibly more. (Under a 2020 Maryland law, criminal charges that don’t result in a conviction are suppressed from the state’s judiciary case search tool.) The office of State’s Attorney Marilyn Mosby cut him a deal in exchange for a guilty plea to two charges: robbery and conspiracy to commit robbery.
 

Although Mr. Walker faced a maximum sentence of 15 years for each count, he received only two years and was freed months before he killed Johnson. He was found guilty in December 2021 of second-degree murder and firearms offenses and sentenced to 50 years in prison.
 

Baltimore’s murders are typically committed by criminals with lengthy rap sheets, who often serve minimal time or are released early. These repeat offenders should be locked up but aren’t, thanks to the dereliction of progressive prosecutors like Ms. Mosby.
 

Using a database of homicide defendants provided by the transparency nonprofit Baltimore Witness, I analyzed the criminal histories of 110 suspects charged with homicide in Baltimore between January 2019 and July 2020. My analysis indicates that the majority of the city’s murders didn’t have to happen.
 

Ninety of the defendants whose histories I examined had previously been convicted of an offense carrying a sentence of three or more years in prison. Most didn’t serve anywhere near that time. They were back on the street when the homicides they are charged with were committed, but they should have been behind bars.
 

More than half (77) of the 110 homicide defendants whose cases I examined were previously convicted of serious crimes under Ms. Mosby. At least 61 of those 77 were convicted of an offense whose eligible jail term, if served in full, would have made it impossible for them to commit their alleged homicides.
 

Ms. Mosby, who decries mass incarceration and decriminalized a slew of quality-of-life crimes by fiat in 2020, makes frequent use of a loophole in state sentencing rules to keep criminals out of prison. Under a “binding plea agreement,” Maryland prosecutors can deem a sentence “compliant” if it falls short of the guidelines range, or even the statutory mandatory minimum term. All that needs to happen is for the prosecutor and defense counsel to agree to the plea before taking it to the judge.
 

At that point, the judge retains veto power only. If the judge accepts the defendant’s guilty plea, the agreed-on sentence can’t be altered. By dropping charges that carry hefty prison sentences, prosecutors like Ms. Mosby can cut offenders loose without running afoul of the rules at all.
 

Baltimore’s police department is overwhelmed. It’s been able to solve and arrest a suspect in only 1 of 3 killings since 2015. But most suspected killers are well known to law enforcement—more than 80% of identified suspects have criminal arrest records, a quarter were on parole or probation at the time of the murder, and in 2019 13% were suspects in prior homicides.
 

In last week’s Democratic primary, Baltimore’s voters delivered their own verdict on Ms. Mosby, who is currently under federal indictment for perjury. With a few ballots left to count, Democrats rejected Ms. Mosby and her policies by a 2 to 1 margin. Her likely successor, Ivan Bates, has promised to reverse most of Ms. Mosby’s policies and tackle the city’s violence by jailing more gun and repeat offenders.
 

That change is welcome, but it will come too late for Justin Antonio Johnson, and for too many Baltimore murder victims who might be alive if their killers had served the prison sentences they earned.
 

Mr. Kennedy is a visiting fellow at the Maryland Public Institute and author of the recent study “Baltimore’s Preventable Murders: The Role of Prior Convictions and Sentencing in Future Homicides.”