Shots were fired from what was described as an armored van in an attack on Dallas Police headquarters early on Saturday, police said, and an explosive device was found outside the building.
Dallas Police Chief David Brown told reporters that a motive for the attack was not yet known. The device later exploded as a police robot attempted to move it.
He said witnesses said as many as four suspects were involved in the incident, which began around 12:30 a.m.
No officers have been injured, but one suspect, who has been speaking with police, said he had been hurt, Brown said.