Gerardo Magallanes is the third person arrested in connection to the killing of Officer Gardiel Solorio.
DOWNEY, Calif. (KABC) -- Two days after a man and a teenager were arrested in connection to the shooting death of a Monterey Park police officer in Downey, police have made an additional arrest.
According to the Downey Police Department, Gerardo Magallanes of San Pedro, 18, was arrested Thursday for murder.
He has since been sent to the Los Angeles County Jail pending his arraignment.
Details surrounding Magallanes' arrest and his alleged involvement in the killing were not immediately released.
On Thursday, 20-year-old Carlos Delcid, who's accused of killing Officer Gardiel Solorio, 26, during what the district attorney called a robbery attempt, made his first court appearance and was ordered to remain jailed without bail pending a September arraignment.
Delcid's murder charge includes a special-circumstance allegation of murder during a robbery.
District Attorney George Gascón said he's facing a possible sentence of life in prison without the possibility of parole.
A 17-year-old suspect - who was not identified - was allegedly driving the pair's getaway car, and was also arrested, Gascón said.
According to the District Attorney's Office, the unidentified teen was charged with a single count of murder in Compton Juvenile Court.
On Thursday, the teen denied the petition - the equivalent of a not-guilty plea in adult court, according to prosecutors. He is due back in court Sept. 14.
Solorio was in his vehicle in the parking lot of an LA Fitness gym in the 12700 block of Lakewood Boulevard around 3:30 p.m. Monday when Delcid allegedly got out of a car nearby and approached Solorio "with a gun drawn in what appeared to be an attempted robbery,'' the district attorney said.
Solorio was found unresponsive in his car, and was pronounced dead at the scene, according to Downey police.
Funeral arrangements were still pending for Solorio, a rookie officer just weeks out of the academy. Monterey Park Police Department Chief Kelly Gordon called his killing "a senseless act of violence.''
City News Service, Inc. contributed to this report.