The Dallas Police headquarters is in lockdown after receiving a threat, and a masked man was spotted nearby.
Police have taken up defensive positions inside the building, and have set up an armed perimeter.
The lockdown follows the armed 'ambush-style' attack on police at a demonstration in which five officers were killed on Thursday night (local time).
Commanders were meeting on Saturday afternoon (local time), when a man wearing a black mask was seen in a parking garage behind the headquarters. The building was surrounded by a SWAT team, who checked nearby cars.
"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures to heightened security," according to a written statement from police.