The gunman who killed five police officers in a sniper ambush in Dallas last week was plotting larger attacks, the city's police chief David Brown has said.
Chilling new details released about Dallas shooter Micah Johnson on Sunday fleshed out a still sketchy portrait of the 25-year-old US Army reservist and Afghanistan war veteran who apparently supported black militant organisations, some classified as hate groups.
Police do not know what the letters mean but believe Johnson wrote them in his own blood, Brown said on CNN. He said Johnson "had been practicing explosive detonations" and possessed enough materials "to have devastating effects throughout our city and our North Texas area".
Police in the US city of Dallas, Texas, stepped up security Saturday after a new threat was received, two days after snipers killed five police officers during a mass protest in the city.
The attack began on Thursday evening during protests over the police killings of Philando Castile, who was fatally shot near St Paul, Minnesota, and Alton Sterling, who was shot in Baton Rouge, Louisiana, after being pinned to the pavement by two white officers.
"I don't think you can hold well-meaning activists. responsible for everything that's uttered at a protest", he said. Johnson also wrote the letters "R, B" in what is believed to be his own blood on the second floor of the El Centro building where police detonated a bomb robot to end the standoff that killed the gunman.
He leapfrogged ahead of demonstrators and stopped when he saw an opportunity to take "high ground" from where he could target police, Brown said.
During negotiations that lasted about two hours, Johnson insisted that he would speak only with an African American police negotiator, Brown said.
The Dallas Police Department said Johnson's Facebook account also included information about Richard Griffin, also known as Professor Griff, who "embraces a radical form of Afrocentrism". "But we can make it harder for them to do so", he said.
"You lose a partner, you just never get over it", Brown said.
Ryan Lenz, online editor and senior writer at the Southern Poverty Law Center, said the number of black separatist groups almost doubled in 2015, mirroring a similar increase among white hate groups that has taken place as police killings make frequent headlines.
Activists with Black Lives Matter, whose peaceful march police were guarding as he opened fire, repudiated the shootings, and it wasn't immediately clear if Johnson had any connection to the movement, which has disavowed violence.
"My family has not only lost a son, but a fellow police officer and a private citizen lost their lives at the hands of our son", Brown told his department at the time.
Mr Obama, who is cutting short his European trip on Sunday to visit Dallas, said that "Americans of all races and all backgrounds are rightly outraged by the inexcusable attacks on police, whether it's in Dallas or any place else".
A woman walks her dog past the home where Micah Johnson, the gunman who killed several police officers at a protest march, lived with his mother Saturday, July 9, 2016, in Mesquite, Texas.
"When we start suggesting that somehow there's this enormous polarisation, and we're back to the situation in the '60s, that's just not true", he added.
Brown also sought to deliver a message to protesters in states like Minnesota and Louisiana after people there were killed by police last week in what critics have called cases of excess force.