Los Angeles County Sheriff Jim McDonnell, with Police Chief Michael Olivieri at his side, confirmed the arrest at a press conference.
"It is with a heavy heart that I must report that one officer did not survive," said Olivieri said earlier on Twitter. "The second officer is in stable condition."
The Los Angeles County coroner's office identified the slain policeman as Greggory Casillas, 30, of Upland.
The fatal sequence of events began shortly after 9 p.m. Friday when the officers responded to a report of a reckless vehicle, and a short pursuit ensued, according to sheriff's Capt. Christopher Bergner.
It is with a heavy heart that I must report that one officer did not survive. The second officer is in stable condition.— Chief Michael Olivieri (@olivieri_ppd) March 10, 2018
After the driver refused to pull over, his truck crashed into a parked vehicle and the officers chased him on foot, Bergner said. The suspect then ran into an apartment in the 1400 block of Palomares Street, near Fernleaf Avenue.
"As he barricaded himself in a bedroom, officers attempted to contact him and he began firing through the door," Bergner said.
Neighbors said they heard a fusillade of gunfire.
The two officers were struck in the shootout; Casillas was shot in the head and was carried out to the street by his colleagues, who attempted life-saving measures.
Both officers were transported to Pomona Valley Hospital Medical Center, where Casillas died. His body later escorted by scores of law enforcement personnel and firefighters to the coroner's office in Boyle Heights while the barricade situation continued at the scene.
The news is tragic this morning as we learn a @PomonaPD officer was shot and killed in the line of duty. Another officer was also shot and wounded. Join us in mourning the loss. Let’s all keep their families and loved ones in our thoughts and prayers pic.twitter.com/53xOy7H5Wy— LAPD HQ (@LAPDHQ) March 10, 2018
The suspect's mangled truck, which sustained significant front-end damage, remained near a curb outside the apartment building as a light rain fell.
Several law enforcement agencies were involved in the standoff with the gunman, including members of the sheriff's SWAT and crisis negotiation teams.
Anyone with information about the incident is asked to call the sheriff's Homicide Bureau at (323) 890-5500.