A woman apparently posing as a police officer broadcast a false report of a hooded man inside the parking garage at Dallas Police Headquarters, prompting a SWAT team response.
Nobody was found and police issued the all-clear, but not before the drama played out on live television.
"There's a guy on the second floor of the parking garage," the woman said in a frantic tone. "You cannot see his face. You cannot tell what he has on."
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Other officers said the voice was not that of a fellow officer but, at first, nobody seemed sure.
"We got units coming to you," a dispatcher responded.
It was unclear how the woman accessed a police radio channel, but Dallas uses older, unencrypted radios.
Earlier, Dallas Police Sr. Cpl. Monica Cordova released the following statement:
"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures to heightened security."
The threat appeared to be separate from the radio broadcast, but heightened tensions in a department and a city already on edge.
There were reports on social media of shots fired and a lockdown of the entire headquarters building.
Police said the reports were not true.
Dallas Rapid Area Transit officials closed the Cedars Station due to the police activity nearby.
Dozens of people have been at DPD headquarters recently to pay their respects at a growing memorial for the officers killed in the line of duty Thursday night in downtown Dallas. Reporters from around the world are also camped out there.
NBC 5's Holley Ford contributed to this report.