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Police released pictures showing gunshot damage to their headquarters
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 suspicious packages were found outside the police HQ, including two bags containing explosives that detonated, police said on Twitter.
The motive for the attack 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 at about 00:30 (05:30GMT) on Saturday. However, he said this had not been confirmed.
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 attempting to negotiate with the occupant of the van, Mr Brown said.
The van, which has been disabled using a police rifle, appeared to have gun ports built into its sides.
The man had identified himself as James Boulware and claimed he was injured, police said.
Mr Brown said police could not confirm his identity, but said that officers had attended three incidents of domestic violence involving a man with that name in the past.
"The suspect has told our negotiators that we took his child and we accused him of being a terrorist, and that he's going to blow us up. And then cut off negotiations," Mr Brown told a news conference.
Suspicious package
Meanwhile, residents near the police headquarters have been evacuated as a precaution as specialist teams dealt with the explosives.
One of the bags exploded as a robot tried to move it.
Dallas police said they successfully detonated another device found under a police truck outside the headquarters, while a third package found in a rubbish container had been cleared and was not an explosive.
Part of the Interstate 45 highway, where the standoff is taking place, has been closed, while nearby rail and bus services have been suspended, AP news agency reports.
Joshua Guilbuad, who lives near the police headquarters, told the BBC he was woken by the gunshots.
"It sounded like approximately 200 gunshots going off across the street."
He said police told him and his flatmates to evacuate the building and shortly afterwards there was "a huge explosion which shook the glass in the windows".