ST. LOUIS – Prosecutors reached a plea agreement on Monday with a 20-year-old man who, as a teenager, shot and killed a retired St. Louis police sergeant in 2018.

Jalynn Garner was set to go on trial this week for the murder of Ralph Harper. Garner had been charged with first-degree murder, first-degree robbery, and two counts of armed criminal action.

Garner previously pleaded guilty to those charges in 2019, but was allowed to withdraw those pleas. At the time, defense lawyers argued Garner pleaded guilty to meet an eligibility deadline for the Dual Jurisdiction juvenile rehabilitation program; Garner was later found ineligible.

The agreement with the St. Louis Circuit Attorney’s Office allowed Garner to plead guilty to second-degree murder; the remaining charges were unaltered.

Garner and Justin Mathews, both Confluence Academy students, were 15 and 16 years old, respectively, when they tried to rob Harper on the morning of Oct. 29, 2018, in the city’s Tower Grove South neighborhood.

The teens approached Harper after he’d parked his car in the 3100 block of Lackland Avenue. Harper, who retired from the St. Louis Metropolitan Police Department in 2007 after more than 30 years on the force, was on his way to babysit two children in his family.

Harper exchanged gunfire with Garner but was fatally wounded. Harper was 67.

Garner was shot in the arm and fled with Mathews in a stolen vehicle. The teens later crashed and were arrested.

Mathews pleaded guilty in 2019 to second-degree murder, first-degree robbery, two counts of armed criminal action, first-degree tampering with a motor vehicle, and resisting arrest by fleeing. A judge suspended Mathews’ 20-year sentence and placed him in juvenile detention under the Dual Jurisdiction program. Mathews will be eligible for parole at 21 if he completes the program. If he doesn’t, he’ll be forced to serve his sentence in prison.