Dallas police officers searched for hours for a suspicious person in the parking garage at their headquarters , the department said. 
The search yielded no suspects or strange items, the department added. 
Officers and police dogs searched the three-story facility for several hours. It was unclear who reported the suspicious person. 
The search came as visitors were dropping by the headquarters to pay tribute at a memorial for five police officers who were killed by a gunman Thursday night after a protest march. 
Dallas police had tightened security throughout the city because of an anonymous threat, spokeswoman Monica Cordova said earlier without elaborating.

Source: QNA