Four people were injured, two of whom were shot, on Tuesday when at least one gunman opened fire at a mall in the San Francisco suburb of San Bruno before apparently fleeing on a nearby train, according to local police.

Two people were taken to nearby hospital who were reported in stable condition, San Bruno Police Chief Ed Barberini told reporters at a news conference.

"It was a senseless act," Barberini said, adding that opening fire in a crowded shopping mall near a Bay Area Rapid.

The search for one or possibly two gunmen who may have fled the scene is ongoing, said the officier.

(With input from Reuters)