A Mattapan man was ordered held without bail Monday on charges he murdered Jean Louis, 20, and injured a teenager on Bailey Street on July 11.

Among the evidence against Tyshawn Pereira, 21: Tracking data from the GPS device he was wearing as a condition of his probation for a 2014 case. Assistant Suffolk County District Attorney Catherine Ham said today data from the device put Pereira on Bailey Street at the time of the murder, showed him running, "at 10 m.p.h." immediately after Louis was shot and then moving away at a more rapid clip in a car.

Michael Johnson-McNeil, 19, and Bobby Robinson-Gilmere, 21 both of Dorchester, were charged as accessories to murder. Dorchester Municipal Court Judge Johnathan Tynes ordered Johnson-McNeil held without bail; Robinson-Gilmere had bail set at $250,000.

According to Ham, Louis and a friend were in a car on Bailey Street around 6 p.m., waiting for another friend to come out of the nearby Ashmont T stop.

Parked nearby, meanwhile, were two BMWs, one blue, one black. As the friend - the 16-year-old - walked down Bailey, two men got out of the BMWs and walked down Bailey as well. As the teen neared the car, at least one of the two opened fire, hitting both the teen and Louis - in his case, fatally - Ham said.

One of the BMWs sped away, then the two men ran up Bailey towards Dorchester, got into the remaining car and sped away. The car was registered to Robinson-Gilmere; the other car to Johnson-McNeil's grandfather, Ham said

Louis's friend, meanwhile, drove him to a local hospital, where he was pronounced dead, she said.

Two days later, Ham said, police spotted the blue BMW near Codman Hill Avenue; it sped away and they broke off the chase, but not before police spotted two guns being tossed out - one from the driver's side, one from the passenger side.

Police found the car abandoned on Codman Hill Avenue, with Pereira's prints on the passenger-side door. Later, GPS data showed him in the same area as the car.

All three men, who hid behind a large door rather than face family members in the courtroom, had not-guilty pleas entered.

Robinson-Gilmere's lawyer pleaded for low bail. He said his client had an alibi for the night of the murder - he was at a cookout on Bailey Street, that prosecutors were "silent" on whether his fingerprints were found on the gun tossed out the driver's side of the car two days later, that he has strong local roots and that he recently had a good job interview at Home Depot.

The three return to Boston Municipal Court on Sept. 23 for a probable cause hearing.