Juan Carlos Mejia, 27, remains at Huntington Memorial Hospital in Pasadena in stable condition.
The crash happened at the intersection of Garfield and Main avenues in Alhambra at about 2:30 a.m. A witness said one officer went into the intersection against the red light and "T-boned" the other officer.
Officer Ryan Stringer was killed in the collision. He had been with the Alhambra Police Department since 2009, and friends and family say he was taken at the prime of his life.
"He was our guardian angel. He took care of our neighborhood," said neighbor Elizabeth Young.
Sonny Stringer, the officer's father, said his son loved his work as an officer.
"He was a people person. He just loved everybody," said father Sonny Stringer. "He was so excited about going to work, he'd be there an hour, hour and a half early."
Stringer was involved in a motorcycle crash just last July while on his way to work on the 605 Freeway. He was placed in a medically induced coma.
"He was not expected to live at that time. They told us he had no chance, then he came back 100 percent. In six months, he was back to work full-time," said Sonny Stringer. "That was July 1st of last year, July 10th of this year, again."
"Everything he did, he had a smile on his face, so, we're going to miss him," Sonny Stringer said.
The last time an officer was killed in the line of the duty at the Alhambra Police Department was 1970.
Gov. Jerry Brown issued a statement offering condolences to Stringer's family. Capitol flags will be flown at half-staff in honor of the fallen officer.
The California Highway Patrol is investigating the accident and has not released any information about the crash.