Friends and fellow officers honored Diaz during a vigil in Lincoln Heights Sunday organized by religious leaders.
LAPD Chief Michel Moore, who attended the vigil, said the death of the 24-year-old officer is a big loss for the department.
"The future of LAPD is what he represented," Moore said. "And everyone I've spoken with, he was a bright smile -- he was a bright mind and he was always willing to give."
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Diaz was off-duty with his girlfriend and her brothers on July 27 eating at a taco stand in the area of Avenue 26 and Artesian Street when he was shot and killed after he saw people tagging a building and told them to stop, police said.
Community residents expressed grief over the death.
"He passed away just being one of us," said Gilma Funaki of Lincoln Heights.
Former police academy classmates say they're proud to have known Diaz.
LAPD has arrested three suspects in connection to Diaz's killing.