The US military veteran who fatally shot five police officers in Dallas, Texas, on Thursday was plotting a larger assault, possibly using explosives, according to authorities.

Micah X. Johnson also taunted police and wrote on a wall in his own blood before being killed by a bomb-disposal robot, Dallas Police Chief David Brown said on Sunday.

"We're convinced that this suspect had other plans," said Brown, adding that the recent deaths of two black men at the hands of police in Minnesota and Louisiana led the Texas shooter to "fast-track" his plans and launch his attack.

He said a search of Johnson's home turned up signs the gunman had practiced using explosives, and that other evidence suggested he wanted to use them against law enforcement targets.

On Friday, five policemen were killed and seven others wounded by Johnson during a protest against police brutality and racial profiling of African Americans.

The attack was the deadliest incident for law enforcement since the September 11, 2001, terrorist attacks.

Before being killed by a bomb-equipped robot, Johnson sang, taunted and laughed at officers, according to the police chief.

The Army veteran also told the officers he wanted to "kill white people" in retaliation for police killings of African Americans.

"He seemed very much in control and very determined to hurt other officers," Brown said.

Over the past few days, thousands of demonstrators have flooded the streets of major cities across the country to decry the deaths of Alton Sterling and Philando Castile at the hands of police officers.

The police killings have once again fueled outrage and widespread protests in the United States, as the issues of racism, gun violence and police brutality shook the country.

Hundreds of people were arrested on Saturday as new protests against the use of deadly force by police flared in several US cities.

Particularly hard hit was St. Paul, Minnesota, where Castile was fatally shot by police last week. Over 20 officers were injured as police were pelted with rocks, bottles and fireworks, officials said.