Multiple gunmen trained automatic weapons on police headquarters in Dallas, Texas, before one man fled the scene in an armoured van, police said on Saturday (13 June).
Police said conflicting witness accounts have made it difficult to establish the exact number of shooters involved.
Following an exchange of fire in the station car park, dozens of squad cars gave chase and the gunman's van was finally surrounded at a parking lot in the nearby suburb of Hutchins.
Dallas Police are currently negotiating with the cornered suspect, and have been providing updates on the situation via Twitter.
A pipe bomb was found in at least one of four bags left outside Jack Evans Police Headquarters.
One of the bags exploded when a bomb squad robot tried to move it, and nearby residents are being evacuated as a precaution.
The motive for the shooting is currently unclear.
Dallas police say no police officers have been injured, although the gunman has told police negotiators that he has been wounded.
He has said he also has explosives in the van, which has gun ports built into its sides.
Dallas News reported that the man has identified himself as James Boulware, 50, who has a history of family violence and blames authorities for losing custody of his son.
Boulware said he was going to blow up police because they "took his child and accused him of being a terrorist", Dallas police Chief David Brown told a news conference.
Authorities have cautioned that they have not yet confirmed the man is who he says he is.