One officer was killed and another seriously injured in Pomona, California, on March 9 after shots rang out while responding to reports of a "reckless vehicle", later leading to a multiple-hour standoff between the suspect and police.
The second officer is in serious condition after being shot in the jaw. Officials didn't identify that officer, but McDonnell said he was shot in the face when he attempted to aid Casillas.
"He left his family at home to protect yours and his ultimate sacrifice will never be forgotten", said Pomona Police Chief Michael Olivieri.
The standoff began at an apartment complex Friday night after the suspect ran from police following a vehicle chase.
Pomona is a city of about 149,000 north of Chino in eastern Los Angeles County.
The driver of the vehicle refused to stop, and a chase ensued, culminating in a traffic collision, Bergner said. The suspect was identified as 39-year-old Isaias De Jesus Valencia, of Pomona.
Those who live in the area of the 1400 block of Palomares Street were asked to evacuate Friday night when a man barricaded himself in an apartment after fleeing police. "As he barricaded himself in a bedroom, officers attempted to contact him and he began firing through the door, striking two officers".
News footage from CNN affiliate KABC shows a heavy police presence, including a SWAT team.
Fifteen hours after the standoff started, Valencia was led away in his underwear, flanked by law enforcement officials. Casillas was finishing his field training when he was killed Friday night, KTLA reported.
Casillas first joined the police department in Pomona, which is 30 miles east of Los Angeles, as a records specialist in 2014.
After Casillas' death, colleagues expressed gratitude for the widespread condolences.
During an afternoon news conference, Olivieri said it was "a sad day" for the Pomona Police Department, community and "law enforcement in general". He is survived by his wife, two young children, parents and two brothers. He was being held without bail.
So far this year, 17 law enforcement officers across the USA have died in shootings, including an officer killed Tuesday in Missouri.