DALLAS — Dallas police officers were searching for a suspicious person in the parking garage at their headquarters Saturday, the department tweeted.
An initial search turned up nothing, and a secondary sweep with police dogs is underway.
The search comes as visitors come by the headquarters to pay tribute at a memorial for five police officers who were killed a gunman Thursday night.
Police said that contrary to media reports, no shots have been fired, but a SWAT team used an explosive device to break through a locked fence.
Police are tightening security throughout the city because of an anonymous threat, spokeswoman Monica Cordova said earlier without elaborating.
Authorities asked news media to stop broadcasting live video from in front of the police headquarters. Vehicle traffic was blocked on the street in front of the building.
Several blocks away, police cars blocked the roads, preventing vehicles from heading toward the station.
Dallas Area Rapid Transit shut down light-rail service to the Cedars Station, which is very close to the headquarters, spokeswoman Morgan Lyons said.
The search also came as Dallas police defended their use of bomb-carrying robot that killed Micah Xavier Johnson, the man who gunned down four Dallas police officers and one transit police officer this week.
The police department said it used the robot as a last resort.
Negotiations had failed and the gunman was still shooting at them, so police used C4 with a detonation cord mounted on a Remotec F-5 robot.
The explosive weighed about one pound, police said.
This story is developing and will be updated when more information becomes available.