(CNN) Dallas police officers searched for hours for a suspicious person in the parking garage at their headquarters Saturday, the department tweeted.

The search yielded no suspects or strange items, the department said.

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.

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.

Ongoing investigation

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.

Obama: Americans are united

Photos: Officers shot during Dallas protest Photos: Officers shot during Dallas protest Police respond after shots were fired in downtown Dallas on Thursday, July 7. Five police officers were fatally shot during a protest over recent police shootings in Louisiana and Minnesota. Seven other officers were injured in the ambush, as were two civilians. Hide Caption 1 of 22 Photos: Officers shot during Dallas protest Emergency responders administer CPR to an unknown patient near the receiving area of the Baylor University Medical Center. Hide Caption 2 of 22 Photos: Officers shot during Dallas protest A police officer with Dallas Area Rapid Transit is comforted at the emergency room entrance of the hospital. Hide Caption 3 of 22 Photos: Officers shot during Dallas protest A police helicopter flies over the scene in downtown Dallas. One suspect was killed by police after a standoff that lasted for hours. Hide Caption 4 of 22 Photos: Officers shot during Dallas protest A man raises his hands as he walks near a law enforcement officer in Dallas. Hide Caption 5 of 22 Photos: Officers shot during Dallas protest Protesters gather as police officers arrest someone in the aftermath of the shootings. Hide Caption 6 of 22 Photos: Officers shot during Dallas protest A Dallas police officer takes a moment as she guards an intersection in the early morning hours. Hide Caption 7 of 22 Photos: Officers shot during Dallas protest Dallas police respond to the scene of the shootings. Hide Caption 8 of 22 Photos: Officers shot during Dallas protest Police officers shield bystanders after shots were fired at the protest. Hide Caption 9 of 22 Photos: Officers shot during Dallas protest Police officers take cover as shots are fired. Hide Caption 10 of 22 Photos: Officers shot during Dallas protest Police get in position after gunshots rang out. Hide Caption 11 of 22 Photos: Officers shot during Dallas protest Two officers crouch behind barriers. Hide Caption 12 of 22 Photos: Officers shot during Dallas protest Dallas police order people away from the area after the shootings. Hide Caption 13 of 22 Photos: Officers shot during Dallas protest Police and others gather at the emergency entrance to the Baylor University Medical Center. Hide Caption 14 of 22 Photos: Officers shot during Dallas protest Police stop a driver in downtown Dallas. Hide Caption 15 of 22 Photos: Officers shot during Dallas protest A view of downtown Dallas after the shootings. Kent Giles captured the image and told CNN he "heard multiple shots being fired. Probably more than 20 rounds. This is the intersection of Main and Griffin looking towards the west." Hide Caption 16 of 22 Photos: Officers shot during Dallas protest Dallas police check a car after detaining a driver. Hide Caption 17 of 22 Photos: Officers shot during Dallas protest Dallas police stand watch after the shootings. Hide Caption 18 of 22 Photos: Officers shot during Dallas protest Onlookers stand near police barricades after the shootings. Hide Caption 19 of 22 Photos: Officers shot during Dallas protest Police attempt to calm the crowd after an arrest. Hide Caption 20 of 22 Photos: Officers shot during Dallas protest A clerk looks at broken windows that were shot out at a store in downtown Dallas. Hide Caption 21 of 22 Photos: Officers shot during Dallas protest Law enforcement officials wait outside the emergency room entrance of the Baylor University Medical Center. Hide Caption 22 of 22

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 victims

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 speeding, Ballwin Police Chief Kevin Scott said. The suspect was arrested and the officer was in critical condition, Scott said.

Saturday, the department identified the officer as Mike Flamion, who joined Ballwin's force in December 2014 after seven years with police in De Soto, Missouri.

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. The man "wanted the police to shoot him" and "he wanted to die," according to the Georgia Bureau of Investigation, citing interviews with suspect.

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 protests

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.

JUST WATCHED Philando Castile shooting aftermath streamed live Replay More Videos ... MUST WATCH Philando Castile shooting aftermath streamed live 02:40

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.

JUST WATCHED Recent police killings reflect a disturbing trend Replay More Videos ... MUST WATCH Recent police killings reflect a disturbing trend 03:07

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.

JUST WATCHED New video shows shooting of Alton Sterling Replay More Videos ... MUST WATCH New video shows shooting of Alton Sterling 01:03

The homeless man called 911 and police arrived at the store, tackled Sterling to the ground, and shot him several times, video shows.