Four officers were shot dead in the US city of Dallas during a rally late Thursday over the recent shootings of black men by police.
"It appears that two snipers shot 10 police officers from elevated positions during the protest rally," Dallas Police Chief David Brown said in a statement quoted by NBC 5.
Three of the fatalities were members of Dallas police, while the fourth was a security official with the local DART transport authorities, the report said.
Police also put out a picture of a "person of interest," it said.
Several shots rang out around 9 pm (0200 Friday GMT) in the city centre, the report said. An 11th officer was injured around 11 pm in a shootout with a suspect, it cited Brown as saying later.
The rally was to protest two killings of black men at the hands of police officers this week.
Philando Castile, 32, was shot late Wednesday near Minneapolis, Minnesota, as he tried to get out his wallet after being pulled over for a broken tail light, police said.
Alton Sterling, 37, was shot Tuesday by police in Baton Rouge, Louisiana, after officers responded to a complaint about a black male selling CDs and threatening the caller with a gun.
The US Justice Department said it was monitoring the state investigation into Castile's killing, and would hold its own into Sterling's death.