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 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 incident at about 00:30 (05:30GMT) on Saturday. However, he said this hadn't 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 negotiating with the occupant of the van, Brown said.
The van has been disabled using a rifle, police said.
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 also said they were investigating a suspicious package under a police truck outside the headquarters, as well as another in a rubbish container.
Joshua Guilbuad, whose apartment is opposite the police headquarters, told the BBC he was woken by the gunshots.
"It sounded like approximately 200 gunshots going off across the street. Then we could see cars pulling up."
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".
"We are now in the basement of the building with no access to food or water," he said.