Police officers in San Francisco are scouring the city for a semi-automatic assault rifle stolen from a police vehicle over the weekend, authorities said.

Unidentified suspects smashed one of the unmarked car's windows and stole an AR-15 rifle while officers worked nearby, Officer Albie Esparza told NBC News.

The high-powered weapon is distributed to trained police officers so that it is at hand for rapid deployment during high-risk events, Esparza said.

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"No resource is being spared to recover this weapon," Esparza said.

Police do not know how the suspects determined the unmarked vehicle belonged to the police department.

The AR-15 assault rifle was the weapon used in last year's movie theater shooting in Aurora, Colo.