Homeland Security Secretary Jeh Johnson said "there appears to have been one gunman" in the shooting.
Investigators discovered bomb-making materials, rifles and a “personal journal of combat tactics” in the shooter's home.
Police cornered one suspect and tried to negotiate with him for several hours but “negotiations broke down” and turned into “an exchange of gunfire with the suspect.”
A "bomb robot" was used to kill the gunman, identified as Micah X. Johnson, 25, a resident of the Dallas area.
Other suspects are in custody, but officials declined to say how many.
In addition to police officers, two civilians -- a man and a woman -- were shot and injured