The suspects in custody have been identified as Francisco Talamantes, 23, Cristian Facundo, 20, and Ashlynn Smith, 18. All three suspects are Temecula residents and are being charged with murder with gang allegation.
Mayor Eric Garcetti and LAPD Assistant Chief Beatrice Girmala confirmed the arrests in brief remarks before a graduation ceremony at the Police Academy in Elysian Park.
WATCH: Mayor, LAPD assistant chief announce arrests in officer's killing
Garcetti described the shooting of the 24-year-old off-duty officer as a "senseless murder," adding that the arrests were the result of more than 100 officers and detectives working round-the-clock shifts.
The mayor noted that Diaz had expressed an interest in becoming a policeman while growing up in Northeast L.A., went on to do "exceptionally well" at the academy and "was doing what a police officer does -- protecting his community" when he was gunned down.
According to the LAPD, investigators served search warrants in Murrieta with the assistance of that city's Police Department in Riverside County. Officers and FBI officials also descended on a home in Glassell Park, where several people were seen handcuffed and detained.
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Diaz was shot and killed after being confronted by a group of graffiti taggers in the early morning hours Saturday while he, his girlfriend and her two brothers were at a taco stand in the area of Avenue 26 and Artesian Street, police said.