(CNN) The calls were fast and frantic as gunshots echoed in the background. "Priority -- shots fired! Shots fired!" a voice said, getting louder with each declaration.

Other voices followed, at times speaking over one another. One repeated the address where gunfire broke out -- over and over.

Police scanner captured the chaotic first few minutes when gunfire erupted while officers were serving a narcotics warrant at a row house in North Philadelphia. On scanner traffic, a flurry of grim announcements came from unidentified officers and dispatch.

"Officer down!" one voice said.

"We need SWAT!" another person yelled.

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