DALLAS, July 7 (UPI) -- Federal and local authorities believe that a brazen ambush-style attack that killed or wounded a dozen Dallas police officers and injured two bystanders was the work of one individual with no criminal rap sheet or known links to terrorism, officials said Friday.
Four officers of the Dallas Police Department and one from Dallas Area Rapid Transit died from wounds they received during the attack. Seven others were wounded, officials said.
Thursday night's shootings occurred during a peaceful rally staged by participants in the "Black Lives Matter" movement, who were protesting the controversial shooting deaths of Alton Sterling and Philando Castile in Louisiana and Minnesota this week.
In the early stages of the investigation, officials suspected that more than one person carried out the attack. At one point, it was believed that at least two sniper-style perpetrators opened fire on the officers from an elevated position near downtown Dallas.
Dallas Police Chief David Brown, though, clarified on Friday that investigators believe a single gunman was responsible. Officials said that suspect, who engaged police in an hours-long standoff that stretched into early Friday morning, wanted to kill white people, and "especially white officers."
Video: Dallas Morning News/YouTube
The suspect, identified as Micah Xavier Johnson, was ultimately killed in a parking garage at a nearby college campus when police sent a robot strapped with an explosive into the area he barricaded himself in and detonated it.
Brown said the gunman had indicated he was the only person involved in the shooting, but authorities continued to investigate the possibility of other suspects. Earlier, the police chief said four people had been taken into custody, but did not give further details.
"We're not expanding on who and how many," he said. "We're going to keep these suspects guessing."
U.S. Homeland Security chief Jeh Johnson echoed Brown's remarks that Johnson was the only gunman.
"At this time, there appears to have been one gunman with no known links to or inspiration from any international terrorist organization," Johnson said Friday afternoon.
Dallas Mayor Michael Rawlings, who also said it appears one person carried out the attack, said the violence shows there are still substantial social differences to overcome.
"We will not shy away from the very real fact that we as city, as a state, as a nation are struggling with racial issues," he said during a prayer vigil.
Despite the lack of terrorist ties, Thursday was the single deadliest day for law enforcement officers in the United States since the Sept. 11, 2001, attacks.
President Barack Obama will cut his weekend Europe trip short and visit Dallas early next week, the White House announced late Friday.
The victims killed in the attack were identified Friday as Dallas P.D. officers Patrick Zamarripa, 32, a Navy veteran who served three tours in Iraq; Michael Krol, 40, an eight-year veteran of the force; Lorne Ahrens, a 14-year veteran; officer Michael Smith, and DART officer Brent Thompson, 43, the first transit agency officer to die in the line of duty.
"As you can imagine, our hearts are broken," DART wrote in a statement. "We are grateful to report the three other DART police officers shot during the protest are expected to recover from their injuries."
"Three DART officers wounded in a Downtown Dallas shootout Thursday night continue their recovery. Ofc. Jesus Retana has been discharged from the hospital. Officers Misty McBride and Omar Cannon are still receiving treatment. DART continues providing support to the family of Ofc. Brent Thompson, who was killed," the agency stated on its website Friday.
Four other Dallas P.D. policemen and three DART officers remain hospitalized with various injuries.
Dallas Area Rapid Transit/TwitterGo Fund Me Account set up in honor of DART Police Ofc Brent Thompson to help his family during this tragic event https://t.co/U7oYJEG96t
— dartmedia (@dartmedia) July 8, 2016
Brown said the gunman told police he was angry about fatal police shootings this week in Louisiana and Minnesota and "he was upset about Black Lives Matter."
"He said he was upset about the recent police shootings," Brown said. "The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers."
Johnson served in the U.S. Army Reserves from March 2009 to April 2015. He was in Afghanistan from November 2013 to July 2014.
Brown said many of the injured officers have been released from the hospital but would not go into detail about the extent of injuries.
Late Thursday, the Dallas Morning News reported at least one suspect was tracked to a parking garage at El Centro College, where he was hiding on the second story, when a shootout took place. Officers attempted negotiations and exchanged gunfire with the suspect.
The attacks came as demonstrators gathered in cities nationwide to protest police shootings of African-Americans in two incidents: Alton Sterling was shot to death on Tuesday by Baton Rouge, La., police after being held down by police officers, and Wednesday Philando Castile was shot and killed by a police officer in Minnesota.
Video: Fort Worth Star-Telegram/YouTube
