Dallas police headquarters in lockdown after threat

Investigators work in the area of downtown Dallas that remains an active crime scene. Photo/AP

The Dallas police headquarters were in lockdown earlier today after an anonymous threat was received.

SWAT teams converged on the area.

"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures to heightened security," Dallas Police said in a statement.

Police spent several hours searching for a suspicious person seen in the parking garage at their headquarters, a spokesman tweeted.

They did not find anyone or any suspicious items.

It is still unclear who reported the suspicious person.

"Search has concluded," Dallas police tweeted.

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

The threat follows the slaying of five Dallas police officer by sniper Micah Johnson.

The 25-year-old old authorities he was upset about the police shootings of two black men earlier this week and wanted to exterminate whites, "especially white officers".