Three men clutching two clown masks and a gun were spotted by a security guard at the Westfield San Francisco Centre Sunday, police said.
The guard was conducting a routine sweep outside one of the mall's entrances near Market and Powell streets when he approached the men before one flashed a gun, police said.
The men, who police say are between the ages of 20 and 30, immediately took off on foot and exited the mall without pointing the weapon at the security guard, according to police.
San Francisco police were dispatched to the scene around 3:15 p.m., but were unable to locate the suspects.
With Halloween just around the corner, San Francisco police are issuing a warning to folks.
"Keep your eyes open for anything that's out of the ordinary and looks suspicious," San Francisco Police Department spokesperson Carlos Manfredi said.
It is unclear if the three men are connected to the recent "creepy clown" hysteria that has been sweeping the nation.
As part of that phenomenon, people sporting clown masks have been discovered scaring people and even threatening Bay Area schools. Although much of the behavior has turned out to be pranks or hoaxes, local law enforcement agencies around the country and the FBI have been keeping a watchful eye.