Armed robbers forced customers and staff to the floor, and then ordered one worker to open the jewellery cases.
The robbers made off with an unknown number of diamonds from the Tiffany & Co. store in San Francisco.
Sgt. Monica McDonald said authorities were viewing footage from surveillance cameras in the Westfield San Francisco Centre area, but remains baffled how the robbers pulled off the heist undetected.
No one was hurt during the robbery.