Snipers operating from rooftops in Dallas, Texas, killed five police officers and wounded six more in a co-ordinated attack during a rally against the killing of two black men by police this week.
Police described Thursday night's ambush as carefully planned and executed, and have taken three people into custody.
A fourth died from what Dallas-based media said was a self-inflicted gunshot after a stand-off that extended into Friday morning.
The fourth suspect had exchanged gunfire with police during the stand-off at a downtown garage and warned of placing bombs throughout the city. Police have yet to confirm his death.
The attack, which took place as a "Black Lives Matter" rally in Dallas was winding up under the watch of officers, was one of the worst mass shootings of police in US history.
President Barack Obama, while in Poland, said he had spoken to Dallas Mayor Mike Rawlings and expressed his "deepest condolences" on behalf of the American people.
"I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas," he said.
"We still don't know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement," he said.
No specific motive has been given for the shootings at the downtown protest, one of many held in major cities across the US on Thursday.
Dallas Police Chief David Brown said the shooters, some in elevated positions, used sniper rifles to fire at the officers in what appeared to be a co-ordinated attack.
"(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," Brown told a news conference, adding a civilian was also wounded.
The suspect in the stand-off had told police "the end is coming" and that more police were going to be hurt and killed.
Brown said the suspect also told police "there are bombs all over the place in this garage and downtown".
Mayor Rawlings advised people to stay away on Friday morning as large areas of the city was cordoned off.
Police were conducting bomb sweeps throughout downtown Dallas, where officials said many county offices were to be closed on Friday.
Federal authorities also halted commercial air traffic in the area as police helicopters hovered over the scene.
The Dallas-Fort Worth metropolitan area is one of the nation's most populous and is home to more than seven million people.
The shooting happened as otherwise largely peaceful protests unfolded across the US after the shooting of Philando Castile, 32, by police near St Paul, Minnesota, late on Wednesday and Alton Sterling, 37, in Baton Rouge, Louisiana, on Tuesday.
Police said they were questioning two occupants of a Mercedes they had pulled over after the vehicle sped off on a downtown street with a man who threw a camouflaged bag inside the back of the car.
A woman was also taken into custody near the garage where the stand-off was taking place.
"We are leaving every motive on the table on why this happened and how this happened," Brown said.
Rawlings visited the wounded at Parkland hospital, the same hospital where President John F. Kennedy was taken after he was shot in Dallas in November 1963.
Outside, officers stood in formation and saluted as bodies of the officers were about to be transported.
