(CNN) A series of brutal assaults in downtown Minneapolis is highlighting the city's skyrocketing robbery rate amid a critical shortage of police officers.

More than 6,000 "priority one" 911 calls, which include sexual assault, shootings and robberies, were made in the period of a year for which police didn't have an officer immediately able to respond, Minneapolis police spokesman John Elder said in an email to CNN.

"Those calls did not get immediately dispatch police resources because we did not have staff available to respond," Elder said.

Robbery is up almost 54% in downtown Minneapolis from last year, police said.

Police have several videos of the recent rash of vicious attacks.

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