The man, dressed in black pants, shirt and beanie with a black and white bandana over his nose and mouth, entered the Wells Fargo Bank at 40982 Fremont Blvd. shortly after 5 p.m. with what looked like a large assault rifle, said Fremont police spokeswoman Geneva Bosques.
"He does a takeover-style robbery where he demands everyone get on the ground, and demands money," Bosques said.
A witness to the robbery called police from outside the bank, and patrol cars arrived within about two minutes, Bosques said.
"Officers were probably one minute or so behind him," she said.
A few blocks away on Almond Avenue, police caught up with a man who said he saw someone drop a pair of latex gloves and a "loot bag."
Police recovered the gloves and bag, which was empty, Bosques said.
An officer then saw and recovered a replica assault rifle in nearby drainage ditch behind the Coach's Sports Bar on Fremont Boulevard.
Police determined that the weapon was an M-4 Crosman air rifle, used to shoot pellets and BBs.
Bosques said that the suspect had covered the tip of the rifle with something like dark-colored fabric, perhaps to prevent people from seeing that it was not a real firearm.
The suspect is described as an African-American male in his 20s, 5 feet 6 to 5 feet 8 inches tall and with a medium build, police said.