Dallas police headquarters were on lockdown on Saturday after receiving a threat against officers, police said two days after a gunman killed five officers in the Texas city.
"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures," to heighten security, Dallas police said in an emailed statement to media.
SWAT officers were deployed at the main building, local media reported.
The sight of a masked man caused police to go on heightened alert, the Dallas Morning News reported. "A man wearing a black mask was spotted in a parking garage behind the headquarters," the paper said.
Officers were reported to be searching the police parking lot.