(CNN) For hours, the barrage of gunfire just wouldn't stop.

Sacramento police Officer Tara O'Sullivan lay on the ground bleeding as gunshots echoed and her colleagues tried to get to her. The suspect fired incessantly, preventing officers from reaching her for at least 45 minutes, authorities said.

O'Sullivan, 26, and her fellow officers had gone to a home in Sacramento after a disturbance call Wednesday and were helping a woman gather her belongings when a man opened fire, Police Chief Daniel Hahn said.

Armed with what appeared to be a high-powered rifle, police said, the suspect blasted rounds every time he saw movement -- at least 30 times over the next four hours. To fend him off, officers fired back more than 100 rounds, Hahn said.

Police vehicles, ballistic vests and ballistic shields were no match for the hail of bullets, so police requested an armored vehicle to rescue O'Sullivan as she lay in the backyard with fatal gunshot wounds.

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