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 have asked news media to stop broadcasting live video from in front of the police headquarters. Vehicle traffic has been 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.
Dallas detectives are piecing together a picture of Johnson to determine whether he acted alone in his ambush of police or had allies in preparing for the shooting that killed the five officers.
Authorities have said they believe Johnson was the sole gunman during the Black Lives Matter protest Thursday in Dallas. What is unclear is whether the 25-year-old army veteran, whom some people described as a loner, conspired with others or how long he had planned an attack.
Seven other officers were wounded in the ambush. Two civilians were also hurt, the Dallas mayor's office said.
The shootings came amid a protest over the fatal police shootings of two black men in Louisiana and Minnesota this week. Similar protests -- mostly peaceful -- were held Friday in dozens of cities across the United States, and more are planned for Saturday.
Obama: Americans are united
President Barack Obama said Saturday the reaction of the majority of Americans, protesters and police to the week's trio of tragedies showed a nation remarkably unified despite some views it is polarized on racial issues.
Americans of all stripes were outraged a "deranged" individual killed the five officers in Dallas just as American have deep concerns about police shootings in Louisiana and Minnesota, said the President, speaking in Warsaw, Poland, where he was attending a NATO conference.
"There is sorrow, there is anger and there is confusion about next steps but there is unity in that this is not how we want our communities to operate, this is not who we want to be as Americans," Obama said
For the President, that was grounds for optimism that the nation would move forward in a constructive way. He noted the scenes around the country were not akin to the polarized 1960s.
"You're not seeing riots and you're not seeing police going after people who are protesting peacefully," the President said.
"As tough, as hard, as depressing as the loss of life was this week, we have a foundation to build on. We just have to have confidence that we can build on the better angels of our nature."
Who was Micah Xavier Johnson?
Johnson had no criminal record or known terror ties, a law enforcement official said.
He served in the U.S. Army Reserve from March 2009 to April 2015, training as a carpentry and masonry specialist, according to Pentagon records. Johnson was deployed for about seven months in Afghanistan, from late 2013, and received an honorable discharge.
Johnson had at least two weapons with him -- a rifle and a handgun, two law enforcement officials have said.
One of the officials, familiar with the latest information from the Dallas police investigation, said the rifle was an SKS semi-automatic. The other official said Johnson legally bought multiple firearms in the past.
Johnson's last known employer was Jeppie Carnegie, who owns a home health care facility in Dallas. He told CNN he hired Johnson in January 2015. Carnegie said a background check showed no past criminal activity.
Johnson's client was his own younger brother and he supported his sibling's home living, Carnegie said. The Touch of Kindness owner told CNN he had infrequent interaction with Johnson, but always as professionals.
The killing of the five officers was the deadliest single incident for U.S. law enforcement since September 11, 2001.
The officers were identified as Dallas Police Officers Lorne Ahrens, Michael Smith, Michael Krol and Patrick Zamarripa, and DART Police Officer Brent Thompson.
Seven officers were wounded. Three of those were DART officers, two of whom have been released. Officer Jesus Retana went home Friday and Officer Elmar Cannon was discharged Saturday. Officer Misty McBride is still in the hospital, DART said on its website.
Other shootings of police
Three other shootings endangered police around the same time.
In Bristol, Tennessee, a man opened fire on motorists early Thursday at a motel along the Volunteer Parkway, killing a woman and wounding three people, including a police officer, the Tennessee Bureau of Investigation said.
The TBI said the suspect, Lakeem Keon Scott, 37, may have targeted individuals and officers after being troubled by recent incidents involving African-Americans and police elsewhere. Scott was wounded by police, arrested and questioned at a hospital, the TBI said.
In metro St. Louis, a police officer was shot in the back of the neck Friday morning by a driver he stopped for a speeding, Ballwin Police Chief Kevin Scott said. The suspect was arrested and the officer was in critical condition, Scott said.
In Valdosta, Georgia, a police officer was shot Friday morning by a man who placed a 911 call to report a car break-in at an apartment building, the Georgia Bureau of Investigation said. The officer, who is in stable condition, was shot once in the abdomen and twice in his protective vest, the GBI said. The officer returned fire and wounded the shooter, who was identified as the caller and arrested.
Man shot in Houston
Houston police said two officers killed a man whom they found standing in the middle of the street brandishing a revolver at 12:40 a.m. (1:40 a.m. ET).
The officers ordered the man several times to drop the weapon , Houston police spokeswoman Jodi Silva said.
They talked to the man when he pointed the weapon in the air but shot and killed him when he pointed the weapon in their direction, she said.
A revolver was found and there are a number of witnesses, Silva said.
The man's race and identify were not immediately disclosed.
The police shooting in Minnesota claimed the life of Philando Castile, a 32-year-old school cafeteria supervisor. After what his fiancee described as a traffic stop for a broken taillight, she streamed the immediate aftermath live on Facebook.
Diamond Reynolds showed Castile groaning and bleeding in the front seat.
"He let the officer know that he had a firearm and he was reaching for his wallet and the officer just shot him in his arm," she said as she broadcast details of the Wednesday shooting on Facebook.
The attorney for Officer Jeronimo Yanez, who shot Castile, said the tragedy resulted from the presence of a gun.
"Regrettably, the use of force became necessary in reacting to the actions of the driver of a stopped vehicle. Officer Yanez is deeply saddened for the family and loved ones of Philando Castile," said attorney Thomas M. Kelly in a statement Saturday.
Baton Rouge shooting
In Baton Rouge, Alton Sterling, 37, was killed Tuesday near a convenience store where he regularly sold CDs and DVDs.
A homeless man approached Sterling and asked for money, becoming so persistent that Sterling showed him his gun, a source told CNN.
The homeless man called 911 and police arrived at the store, tackled Sterling to the ground, and shot him several times, video shows.