Dallas police sent out an alert Saturday about an unspecified threat and asked news media not to broadcast live near the headquarters for the safety of officers.
"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 Reuters.
Within an hour of the start of the lockdown, police declared the search concluded after searchers of the garage, first, second and third floors by SWAT teams found nothing suspicious.
Several officers were spotted rushing toward a department parking garage. But members of the public were still able to walk about freely around the building. The police department said police headquarters is not in lock down, disputing some reports in the media.
The Dallas Morning News, citing a police source, reported that a lockdown was in effect shortly before 6 p.m. local time.
Johnson, who was himself killed during the assault, supported black militant groups who encouraged violence against police.
Clarence Castile was standing at the site where Philando Castile was shot and killed by a police officer last week.
He said Americans of "all races, all backgrounds", including many of those who were protesting, were outraged by the Dallas killings.
At least two Dallas Police Department buildings are now blocked off, with numerous police vehicles and armed officers outside. Two civilians were also hurt.
The man identified as 25-year-old Micah Johnson told authorities as he was dying that he was upset about the police shootings of two black men earlier in the week and wanted to exterminate whites, "especially white officers", officials said. It was being called a "justice rally" and was part of a nationwide protest following officer-involved shootings in Louisiana and Minnesota.
Leaders of the Black Lives Matter organisation have condemned the Dallas killings but say planned marches, including a "Weekend of Rage" in Philadelphia, will go ahead.