On Wednesday, the bank was continuing to turn customers away as the investigation into the incident continued.
Neighbors and witnesses were left shaken by the deadly encounter.
"I heard yelling and I was like what is that?" said Julie Riley, who lives near the bank. "Then I heard the gunshot. I was right there, scared me half to death."
The suspect was not carrying identification and authorities have yet to officially confirm his identity.
UPDATE: Fullerton police later identified the suspect and released body camera video and 911 audio on this incident.
The incident happened around 5 p.m. Tuesday, as employees at the Wells Fargo branch on Bastanchury Road in Fullerton were preparing to close down for the day.
At least one customer was still inside when a man wearing a mask entered and produced a white box that looked like an explosive device, police say.
He demanded money, threatening to blow up the device if he wasn't paid.
He put the money into the box and walked outside, where he was confronted by officers. The exact interaction hasn't been disclosed in detail, but officers ended up opening fire, killing him.
They did not approach his body at first, waiting for the bomb squad to respond and examine the device first. They determined it was "an item constructed to resemble an actual explosive device," police said.
Eventually he was pronounced dead at the scene.
"A bank robbery - no matter the age or the way they're doing it - is something that is not only scary for officers responding, (but) scary for the employees and patrons inside the bank," said Fullerton PD Sgt. Ryan O'Neil. "It hits the community, too."
Per state law, the California Department of Justice is reviewing the officer-involved shooting.