AP Photo/Tony Gutierrez
"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.
Police headquarters have been placed on lockdown and are surrounded by SWAT team vehicles, according to local media, which cited unnamed sources.
The department said it would not immediately confirm or deny whether its headquarters was under lockdown.
WFAA reporter Jason Whitely said police were looking for a man near the department's headquarters, "posing some kind of threat."
DPD Sgt. Warren Mitchell told Dallas Morning News reporter Naomi Martin that no shots have been fired.
(Editing by Sandra Maler)