Tributes paid to slain police officers as details emerge about suspect Micah Xavier Johnson
A gunman involved in a deadly attack in Dallas that killed five police officers said he was "upset" at recent fatal police shootings of black men.
The alleged attacker, identified as Micah Xavier Johnson, told a police negotiator he was upset at white people and wanted to kill them, especially white officers, before he was killed by a robot-delivered bomb.
Dallas police chief David Brown described the shootings, which also left seven officers and two civilians wounded, as a "well-thought-out evil tragedy".
Johnson had served in the Army Reserve for six years - and completed a tour of duty in Afghanistan between November 2013 and July 2014.
Before he died, the 25-year-old said he was acting alone and was not affiliated with any terror group.
The "ambush style" attack began at around 8.45pm on Thursday, as hundreds of people gathered to protest over fatal police shootings this week in Baton Rouge, Louisiana, and suburban St Paul, Minnesota.
During the stand-off, Johnson told police that "the end is coming" and there were bombs "all over downtown Dallas".
However, investigators said no explosives were found following "extensive sweeps" of the area.
Last video I got. Sorry for the language we were scared and confused. pic.twitter.com/o0LBivBg8v— Allison (@allisongriz) July 8, 2016
It was the deadliest day for US law enforcement since the 9/11 terrorist attacks in 2001, according to the National Law Enforcement Officers Memorial Fund.
Police had earlier said at least two "snipers" committed the shootings, but it remains unclear how many attackers were involved.
Two men and a woman were being questioned in connection with the attack, and Johnson's home in a suburb of Dallas - about an hour away from the scene of the shootings - has been searched by investigators.
Meanwhile, two of the victims have been named as 43-year-old Brent Thompson, who worked for the city's transport police and had only recently got married, and Officer Patrick Zamarripa of the Dallas force.
Another of the victims was Michael Krol, who used to work at the Wayne County Sheriff's Office.
DART grieving the loss of Ofc Brent Thompson, 43, killed during Thurs protest. First DART officer killed in line of duty. Joined DART 2009.— dartmedia (@dartmedia) July 8, 2016
Mr Brown said the negotiator had done an "exceptional job" in getting the gunman to talk before he died.
The police chief added: "He said he was upset about the recent police shootings.
"The suspect said he was upset about Black Lives Matter.
"The suspect said he was upset at white people.
"The suspect stated he wanted to kill white people, especially white officers."
Mr Brown revealed the negotiations had broken down after an exchange of fire between the gunman and officers and they "saw no other choice" but to use a robot to detonate an explosive device next to him.
He said: "Other options would have exposed our officers to grave danger."
Image: Tony Gutierrez / AP/Press Association Images
Mr Obama, who had been briefed on the shootings during a visit to Poland, said every American would be "horrified" at the "senseless murders".
Paying tribute to the police, he added: "Today is a wrenching reminder of the sacrifices that they make for us."
He went on: "We also know that when people are armed with powerful weapons, unfortunately it makes attacks like these more deadly and more tragic.
"And in the days ahead we are going to have to consider those realities as well."
The attack came amid protests across the US over recent police shootings.
On Tuesday, music vendor Alton Sterling was shot dead in Baton Rouge, Louisiana, after being held down by police officers.
The following day, Philando Castile was in a car with his girlfriend and child when he was shot dead by an officer in Minnesota.
Both shootings were captured on mobile phones and the footage widely shared.