Officers are in a stand-off with one or more shooters after the police headquarters in Dallas, Texas came under fire.
Explosives were later found around the building, the police department said.
A van, described by witnesses as an armoured vehicle, rammed into police squad cars at the headquarters just after midnight on Saturday local time, then opened fire, Dallas police chief David Brown told reporters.
A chase ensued to a nearby suburb south of the city where officers have surrounded the vehicle, which contains at least one suspect, Brown said, adding that up to four individuals may have been involved in the headquarters attack.
He said pipe bombs had been found in one of four duffel bags that were "dispersed throughout the front and side of police headquarters".
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The police department said on Twitter that one of the bags "exploded on its own" as a bomb disposal robot attempted to move it, and that another device had been found under a police vehicle and detonated.
No police staff have been injured so far in the stand-off, Brown said.
