The police headquarters in the US city of Dallas, Texas, has come under attack by an unknown number of gunmen who opened fire on officers.
One suspect fled the scene in what was described as an armoured van and is involved in a standoff in a car park.
A number of bags were found outside the police HQ, at least one of which contained explosives including pipe bombs, police said on Twitter.
The motive for the shooting is not clear. No officers have been hurt.
Dallas Police Chief David Brown quoted witnesses as saying that as many as four suspects were involved in the initial shooting incident in the early hours of Saturday.
He said one suspect then fled the scene in the van, ramming a police car.
A police chase involving dozens of squad cars ended in a car park in Hutchins, about nine miles (14km) from central Dallas, and officers are negotiating with the occupant of the van, Mr Brown said.
Residents near the police headquarters have been evacuated as a precaution as specialist teams dealt with the explosives.
One of the bags exploded as experts tried to move it.
Dallas police also said they were investigating a suspicious package under a police truck outside the headquarters, as well as another in a rubbish container.
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