The US military veteran, who fatally shot five Dallas police officers in a racially motivated attack last week, asked negotiators how many people he had shot and told them he wanted to kill more, the city's police chief said today.
Explosives found at the home of gunman Micah Johnson suggested he had been plotting a larger assault, said authorities, who were still trying to understand a message he wrote in his own blood on a wall before being killed by a bomb-equipped robot sent in by the police.
"We knew through negotiations this was the suspect because he was asking us how many did he get and he told us how many more he wanted to kill," Dallas Police Chief David Brown told reporters today.
The attack on Thursday night came at the end of a demonstration over police shootings that had been prompted by incidents earlier in the week in which police in Baton Rouge, Louisiana, and St. Paul, Minnesota, killed two black men.
Johnson, a 25-year-old African-American, told police negotiators during an hours-long standoff that he had been angered by those deaths and had wanted to "kill white people."
The deaths in Baton Rouge and St. Paul were the latest in a series of high-profile and controversial killings of black men by police in cities including New York, Ferguson, Missouri, Chicago and Baltimore.
Even as officials and activists condemned the shootings and mourned the slain officers in Dallas, hundreds of people were arrested on Saturday and Sunday as new protests against the use of deadly force by police flared in U.S. cities.
Scores of people were arrested in Baton Rouge on Sunday after authorities warned that violence during street demonstrations would not be tolerated.
Brown said a search of Johnson's home showed the gunman had practiced using explosives, and that other evidence suggested he wanted to use them against law enforcement officers.
His attack was the deadliest day for US law enforcement since the hijacked plane attacks of Sept. 11, 2001. Nine law enforcement officers were also wounded, Brown said on Monday.
Disappointed with military
Johnson, who had served with the US Army Reserve and had been deployed in Afghanistan, had been "disappointed" in his experience with the military, his mother told TheBlaze.com in an interview broadcast online today.
"The military was not what Micah thought it would be," Delphine Johnson told The Blaze. "He was very disappointed. Very disappointed." She did not give details.
Several media organisations have reported that while Johnson was in Afghanistan from November 2013 to July 2014, a woman soldier in his unit accused him of sexual harassment.
The US Department of Defense and a lawyer who had represented Johnson in the past did not return requests for information on his military history or the status of his discharge.
In Baton Rouge, protesters faced off with police officers wearing gas masks on Sunday evening. Media, citing Baton Rouge police, reported that at least 48 people were taken into custody after demonstrators clashed with police following a peaceful march to the state capitol.
In St. Paul, Minnesota, 21 officers were injured on Saturday when they were pelted with rocks, bottles, construction material and fireworks.
Three countries have warned their citizens to stay on guard when visiting US.cities rocked by the protests.
A candlelight vigil was set for 8 p.m. today in Dallas City Hall plaza. President Barack Obama was due to travel to the city on Tuesday to attend a memorial for the slain officers.