Dallas police find explosives

US authorities released more information on Friday about Micah Xavier Johnson, the gunman who shot five police officers dead and wounded seven more in Dallas on Thursday. Police said they found bomb-making materials as well as more guns and ammunition in Johnson's home in a Dallas suburb.

Mayor Mike Rawlings also confirmed that investigators believe Johnson was acting alone.

Johnson open fired during a peaceful protest against police brutality in the wake of the deaths of two unarmed black men at the hands of officers earlier in the week. Philando Castile's death in Minnesota sparked nationwide outrage as his girlfriend live-streamed an officer shooting him as he reached for his driver's license. Alton Sterling was killed outside a shop in Louisiana.

Use of remote bomb begs questions

Johnson was eventually killed by a remotely-delivered bomb after hours of standoff and failed negotiations with officers. Dallas Police Chief David Brown said Johnson had told negotiators that he wanted to kill white police in retaliation for the deaths of innocent African Americans.

The use of the bomb robot, thus far unprecedented, has not been without criticism amidst rising fears of the militarization of municipal police forces in the US.

"We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was," Brown said.

"Other options would have exposed officers to grave danger," he said, without elaborating as to why other strategies were not used.

Johnson was identified on Friday as an army veteran who served one tour in Afghanistan from late 2013 to mid-2014. He wore a protective vest and used an AR-15 rifle, a weapon that has been used in mass shootings in the US.

Politicians and public figures expressed the need for solidarity across racial lines and between officers and civilians. Attorney General Loretta Lynch urged activists like the Black Lives Matter movement not to give up their peaceful protests, saying that they should not be deterred "by those who use your lawful actions as a cover for their heinous violence."

Similar attacks in 3 US states

Possible copycat attacks were also reported in three other US states on Friday. Officials said a man in Georgia called the emergency services to report a break-in, then ambushed police when they arrived. Both the suspect and an officer were wounded and taken to hospital, but were expected to survive.

In Tennessee, a man who was arrested after allegedly shooting at random passing cars and police on the highway told authorities he was fed up with violence against African Americans. Another incident occurred in Missouri, when a driver shot an officer at a traffic stop after his back was turned. The officer remained in critical condition, police said.

Rallies and vigils

Thousands of people marched in Atlanta, Georgia to protest the shootings of black citizens by white police officers in Minnesota and Louisiana. Protesters held signs and chanted "hands up, don't shoot."

An interfaith service was held in Boston to pray for an end to racially tinged violence. There were also rallies in Arkansas, Los Angeles, Omaha, Pittsburgh, Denver, Washington DC and in London, England where hundreds of people took part in a Black Lives Matter protest on Friday.

Obama visit

President Barack Obama has cut short his European tour to Poland and Spain and is to return to Washington on Sunday night. He is due to visit Dallas at the request of Mayor Mike Rawlings early next week.

Obama has aligned himself with civil rights protesters and others calling attention to racial disparities in the justice system.

Obama ordered flags flown at half-staff in honor of the Dallas victims and he spoke by phone to Rawlings, Dallas Police Chief David Brown and Attorney General Loretta Lynch on Friday. He offered his condolences and federal support to the local officials, the White House said.

es/jm (AP, Reuters)