The mother of the son of a black man killed by white Louisiana police officers said Friday she grieved with the families of five police officers killed in Dallas during a protest over police shootings, adding she was now.

Although Johnson said he wanted to kill white police officers because of recent police shootings of black men, Dallas has one of the more diverse police forces in the USA and it has reportedly made steps to improve its relationship with the black community.

His brother-in-law said he recently had a talk with Smith about retirement, but said the officer was more concerned about his fellow officers and remained on the force.

Seattle Police Chief Kathleen O'Toole said she wasn't aware of the post when asked about it Thursday.

During an overnight news conference, Dallas Police Chief David Brown described Hughes, without naming him, as "a person of interest who witnesses at the scene say was involved in the shooting". When the officer arrived, the man, Stephen Paul Beck, 22, shot him several times, the police said.

Mayor Mike Rawlings said the dead suspect was killed by police using explosives in a parking garage where the man had exchanged gunfire with officers.

The Army has confirmed that Johnson served as an enlisted soldier and served a tour of duty in Afghanistan.

Brown said three other people were in custody but didn't provide any additional details.

"Why do so many whites (not all) enjoy killing and participating in the death of innocent beings", Johnson, 25, wrote on Saturday above a video of what appeared to be people participating in a whale-killing.

At the request of West, attendees joined hands as they pledged to work toward a solution to the rampant violence and racial tension, including the murder of Alton Sterling and Philando Castile at the hands of police earlier in the week.

Chief Jon Belmar said Friday at a news conference that the suspect, who police say shot the officer as he was walking back to his auto during a traffic stop, also had been on probation for a stolen vehicle in Oklahoma and was picked up for a firearm violation in California.

Hashtags #PrayFor Dallas, #BlackLivesMatter, #Castile and #Sterling trended globally.

Seven officers were injured in the shootings.

Mr Brown said police do not yet have a motive for the attacks or any information on the suspects.

The search for the shooters stretched throughout downtown, an area of hotels, restaurants, businesses and some residential apartments.

"Everyone just started running", Devante Odom, 21, told The Dallas Morning News.

"Officers get ready every day". It was tap, tap pause.

Other protests across the US on Thursday were peaceful, including in New York, Atlanta, Chicago and Philadelphia. "We're determining how big that crime scene is", the mayor said. In midtown Manhattan, protesters first gathered in Union Square Park where they chanted "The people united, never be divided!" and "What do we want?"

In Poland for a North Atlantic Treaty Organisation summit, President Barack Obama called the shooting "a vicious, calculated and despicable attack on law enforcement".

Speaking from Warsaw, Poland, where he was meeting with leaders of the European Union and attending a North Atlantic Treaty Organisation summit, the president asked all Americans to pray for the fallen officers and their families.