Three alleged gang members charged with killing a Downey police officer last November now face additional counts including a string of armed robberies and shootings, some just hours before the officer was gunned down, the Los Angeles County District Attorney's Office announced today.

Prosecutors also added a fourth defendant to the case, the girlfriend of one of the defendants who was involved in an attempted murder a month before Officer Ricky Galvez, 29, was fatally shot on Nov. 18, 2015.

Prosecutors charged defendants Steven Knott, 18, and Jeremy Anthony Alvarez, 21, with an armed takeover robbery at a cell phone store in Bellflower that occurred around 5 p.m. on the same day they're charged with fatally shooting Officer Galvez. The five-year veteran of the Downey Police Department was shot and killed as he sat in his personal car in a parking lot adjacent to the police station.

And less than an hour before they allegedly killed the officer, Knott, Alvarez and Abel Diaz, 17, are charged with using a firearm during a street robbery of a man in Paramount.

In addition to the murder and attempted robbery charges filed in the Galvez case, Knott is now charged with shooting at a South Gate residence on San Gabriel Avenue on Sept. 17, 2015. The incident occurred after Knott allegedly got into a verbal argument with an occupant.

Knott also is charged along with Alvarez and Alvarez's girlfriend, Rebecca Ann Ustrell (dob 12/8/87) with the Oct. 18, 2015, attempted murder of a man shot during a verbal dispute at a Montebello convenience store.

The four defendants are back in court today in Norwalk, Department H, to set a date for their preliminary hearing in case VA140624.

All four defendants have pleaded not guilty. Knott, Alvarez and Diaz are being held without bail. Ustrell is being held on $1 million bail.

Deputy District Attorneys Michael Blake and Mary Murray with the Crimes Against Peace Officers Section are prosecuting the case.

If convicted, Knott and Alvarez could face the death penalty if prosecutors decide later to seek it. Diaz, because of his age, faces up to life in state prison if convicted as charged. Ustrell faces up to life in state prison.

The case remains under investigation by the Los Angeles County Sheriff's Department.

Los Angeles County District Attorney / case VA140624 / August 2, 2016