Dallas police have given the all-clear after an anonymous threat against officers sparked a floor-by-floor search of the city's police headquarters. The Dallas Police Department (DPD) said in a statement that a Swat team and dog units were deployed to the building's parking garage after a "suspicious person" was spotted nearby.

The search which involved an explosive and a shotgun to gain access to locked passages did not turn up anyone.

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It comes two days after five white police officers were shot and seven others wounded by Micah Johnson – a black man – at a peaceful demonstration against police brutality in the city. It was the single deadliest incident for US law enforcement since the 9/11 terror attacks in 2001.

"Out of an abundance of caution, officers searched the garage to ensure reports of a suspicious person was thoroughly investigated," Dallas police said.

Racial tensions

Officers told the AFP news agency that the police force's headquarters was placed on lockdown after an anonymous threat was received, but the DPD insisted that this was not the case. "There has been media reports of shots fired at police HQ, there has been no shots fired. Swat set off a device to enter a locked fence," it said. "Officers have completed manual search of the garage. No suspect found."

Dallas police chief David Brown congratulated his officers on a "great job".

Earlier, the Dallas Morning News reported that a masked man was spotted in the premises of the main police headquarters. Police requested media organisations to "stop all live feeds of DPD headquarters" while the search was ongoing "for the safety of our officers".

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Dallas police chief David Brown attended a prayer vigil for the five slain officers on 8 July Spencer Platt/Getty Images)

Thousands marched in cities across America on 9 July to protest against the fatal shootings of two black men by police in Louisiana and Minnesota, over the past week. President Barack Obama denied that the killing of Dallas police officers on 7 July represented a new low in race relations in the US.

"As painful as the week has been, I fully believe that America is not as divided as people have suggested," he was quoted as saying by USA Today on the sidelines of a Nato summit in Warsaw.

"There is sorrow. There is anger. There is confusion about the next steps. But there is unity in recognizing that this is not how we want our communities to operate. This is not who we want to be as Americans," he said.

Barack Obama