Five Dallas police officers were killed and six wounded last night when a sniper opened fire during a protest against the recent shooting of two black men by police.
Authorities are currently engaged in an armed stand-off with a man in a multi-storey car park, Dallas police chief David Brown said.
The shootings took place at 8:45pm local time on Thursday amid one of several demonstrations across the US against the killings earlier this week of Philando Castile in Minnesota and Alton Sterling in Louisiana.
Brown said: "The suspect that we are negotiating with has told our negotiators that the end is coming and he is going to hurt and kill more of us, meaning law enforcement, and that there are bombs all over the place in this garage and in downtown."
The police chief earlier said that 11 officers were shot "ambush style" by the sniper, with three being killed. Dallas police later confirmed that a fourth officer had died and the Dallas Police Association subsequently said that a fifth had died.
added that a woman is now being questioned and that they had intercepted a car after a person threw a camouflaged bag into the back of it before speeding off. Two occupants of the carwere being questioned.
Brown said: "We do not have a comfort level that we have all the suspects...We believe that these suspects were positioning themselves in a way to triangulate on these officers from two different perches... and planned to injure and kill as many law enforcement officers as they could."
Rev Jeff Hood, an organiser of the march, told the Dallas Morning News: "I ran away from the shots trying to get people off the streets and I was grabbing myself to see if I was shot."
Mike Rawlings, the mayor of Dallas, said that the shootings marked a "heartbreaking moment for the city".
The violence came as several protests were held across the US in response to police use of lethal force against African Americans, in incidents that were caught on video, sparking a national debate about racism in the police.
On Wednesday, Philando Castile was shot dead at a traffic stop in St Paul, Minnesota and Alton Sterling was killed the previous day in Baton Rouge, Louisiana.
President Barack Obama said that "all fair-minded people should be concerned" about the apparently disproportionately frequent killings of black Americans by police.
"When incidents like this occur, there's a big chunk of our fellow citizenry that feels as if it's because of the colour of their skin, they are not being treated the same,'' he said. "And that hurts."