A threat against Dallas police and the sighting of a suspicious person in a parking garage near their headquarters put officers on high alert Saturday afternoon, two days after they came under fire in downtown Dallas.

Officers searched the garage after using a shotgun to break a lock on a fence there, but the hunt turned up nothing, as did a second search by police dogs. Other media reported that the person may have fled.

An early report of gunfire was debunked by police, who also said there was never a lockdown at the headquarters.

But police were clearly taking no chances, especially in light of the deadly assault on Thursday night. Security was tightened at police buildings across the city.

A crowd of onlookers gathered on the sidewalk across from headquarters while the garage was being searched. Others peered out windows and stood on balconies overlooking the scene.

Many had come down to a makeshift memorial to pay their respects to fallen officers and stayed to watch the situation unfold. Some were unable to leave, their parked cars blocked in by the closure of Lamar Street.

"I think this is going to be the norm for a while. Everyone's on high alert" Blake Courtney said from the sidewalk. Courtney, who lives in the Southside on Lamar apartment complex, remembers watching the attack on police headquarters a little more than a year ago.

"I think it's messed up," he said of Thursday's attack. "There's this sense of disbelief."

Dave Braeger, a professional artist, sat on the stoop outside an apartment across from the police parking garage. He was holding a painting he'd made that featured a depiction of Reunion Tower with the words "Blue Lives Matter".

"I wanted to give this as a gift to the officers," Braeger said. "The problem I have is with this national bashing of police officers. There's no question that 99 percent of officers are wonderful and keep us safe."

Earlier Saturday, police commanders met at headquarters to update Chief David Brown on the latest in the investigation. After the meeting wrapped up, Brown told investigators to go home and get some rest because they had been working tirelessly since the attack.

Officers in the building were on high alert shortly afterward, after police received an anonymous threat. Lamar was shut down, as was the nearby Cedars DART station.

Not long after that, there was a report of a suspicious man, possibly wearing all black, in the garage behind headquarters, and SWAT officers surrounded the building with guns drawn.

The parking garage is gated, so it would be difficult to get inside in a vehicle without an access card. But it would be possible to enter the garage on foot.

Mourners were visiting a makeshift memorial in front of headquarters as SWAT officers moved toward the back of the building to secure the parking garage. One mourner knelt down and began shouting prayers as police raced around.

SWAT officers checked cars near the headquarters and went through nearby dumpsters.

1 / 11Dallas police officers searched a parking garage near their headquarters near downtown Dallas on Saturday. (Rose Baca/The Dallas Morning News)(Rose Baca / Staff Photographer) 2 / 11A Dallas police officer on alert at Dallas Police Headquarters where a credible threat was recieved on Saturday, July 9, 2016.(The Dallas Morning News) 3 / 11Dallas police officers respond to a report of a threat at the Dallas Police Headquarters on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 4 / 11Dallas police officers respond to a report of a threat at the Dallas Police Headquarters on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 5 / 11Dallas police officers -- one with his gun drawn -- respond to a report of a threat at the Dallas Police Headquarters on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 6 / 11A DPD police officers carry heavy armor into the Dallas Police Headquarters on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 7 / 11A DPD police officers carry heavy armor and rifle bag into the Dallas Police Headquarters on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 8 / 11FBI investigators on scene collecting evidence from the Thursday night shooting killing 5 officers and injuring another 7 at the cross section of Lamar St. and Main St. on July 9, 2016 in Dallas. (Ting Shen/The Dallas Morning News)(The Dallas Morning News) 9 / 11Dallas Police stand guard along Belleview Street near Dallas Police Headquarters in Dallas on July 9, 2016. (Rose Baca/The Dallas Morning News)(Rose Baca, The Dallas Morning News) 10 / 11Dallas Police stand guard along Belleview Street near Dallas Police Headquarters in Dallas on July 9, 2016. (Rose Baca/The Dallas Morning News)(Rose Baca, The Dallas Morning News) 11 / 11Dallas Police stand guard along Belleview Street near Dallas Police Headquarters in Dallas on July 9, 2016. (Rose Baca/The Dallas Morning News)(Rose Baca, The Dallas Morning News)

Officers, including homicide and robbery detectives who work evenings, are inside the building.

"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures to heightened security," according to a written statement from police.

The doors to the complex on South Lamar Street were locked and officers could be seen inside taking a defensive stance at the doors.

SWAT officers were dispatched to HQ, and a police source said a lockdown was in effect, but that wasn't confirmed in the police statement.

WFAA-TV (Channel 8) was reporting the threat came from an armed group in Houston.

The threat comes two days after a lone gunman killed five officers and wounded several more in a sniper attack in downtown Dallas.

The headquarters building came under fire itself in June of last year when a man in an armored vehicle attacked police.

Staff writers Tasha Tsiaperas, Naomi Martin, Conor Shine, Austin Huguelet and Robert Wilonsky contributed to this report.