Authorities said the shooting happened in the 900 block of West Holt Avenue about 8:30 a.m.
Pomona police said they were patrolling the alley behind the Pomona Inn Hotel when they encountered a Chevy Impala with a man and female inside. Police said the man got out of the car and became violent.
"During the detention, a male became violent and an officer-involved shooting occurred," said Lt. Holly Francisco of the Los Angeles County Sheriff's Department, which is handling the investigation.
The man was declared dead at the scene.
The driver side backseat window was shot out and a bullet struck the building.
"I heard shots, I ran outside and there were two cops just standing there and they were putting my sister in the back of the car," said Adrien Huguez, who said his sister was inside the car. "My sister was crying. You could just see the feet of a body laying behind my sister's boyfriend's car."
Lisa Ortiz said her daughter's boyfriend was not armed.
"You don't kill innocent people when all the bad people are still out there in the street," Ortiz said. "Killed my daughter's boyfriend with my daughter in the car in cold blood."
It's not yet clear why officers approached the man and his girlfriend.