A man in a mask and combat gear was fatally shot Monday morning in downtown Dallas after he opened fire with an assault weapon outside the Earle Cabell Federal Building. No one else was injured.

FBI Special Agent in Charge Matthew DeSarno identified the gunman as Brian Isaack Clyde, 22, at a news conference on a street corner near the federal building. Clyde died at the scene and was taken to Baylor University Medical Center, officials said.

Neither DeSarno nor Erin Nealy Cox, U.S. attorney for the Northern District of Texas, gave any indication why Clyde targeted the federal building. They also did not say who shot Clyde after police responded to an active-shooter call.

An armed shooter (shown) attacks at the Earle Cabell federal courthouse Monday morning in downtown Dallas. Law enforcement returned fire and the shooter was hit by gunfire. No officers or citizens were injured. FBI Special Agent in Charge Matthew DeSarno identified the shooter as Brian Isaack Clyde, 22 (Tom Fox / Staff photographer)

"We're looking into motive," DeSarno said at an afternoon news conference. DeSarno said the FBI had not investigated Clyde before the shooting and he was not on any watch list. Investigators were "aggressively pursuing" his social media presence, the agent said.

Dallas Morning News photographer Tom Fox saw Clyde fire outside the building on Jackson Street and took photos as the shooting occurred.

Brian Clyde is seen in two photos posted to his Facebook page, most recently in the one (right) posted May 8, 2019. Clyde opened fire at the Earle Cabell Federal Building in downtown Dallas. (Facebook)

Fox said Clyde fired from the parking lot across the street toward him, another man, a security guard and a woman who was walking a golden retriever.

The windows in a revolving door and two side doors at one entrance were broken. It was unclear whether Clyde or law-enforcement personnel had shot the door.

Watch exclusive video from Dallas Morning News photographer Tom Fox, who witnessed the shooting:

Watch the shooting unfold in a video taken by a witness, Tim Brown:

Fox's photos show authorities surrounding Clyde as he lay in a parking lot where he'd run and fallen after the shooting.

In one photo, a Homeland Security agent wearing blue latex gloves is hovering over Clyde. In others, Clyde is shirtless and law enforcement officers, including the agent, kneel around him. On Clyde's left arm, he had a red heart tattoo with the silhouette of a cat inside it.

Fox, who was questioned by the FBI, said he had been outside the building when Clyde parked on the corner of Jackson and Griffin streets. Clyde ran and then stopped on the sidewalk to pick up a loaded magazine he had dropped.

He then began shooting at the courthouse along Jackson Street as security personnel on the first floor pushed people to the ground. At least two bullets ricocheted off the building, spreading clouds of dust and debris.

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The federal building houses federal courts, the U.S. attorney's office for the Northern District of Texas, a passport office and the U.S. Marshals Service. Streets around the courthouse will be closed for several days, Dallas police said.

Nearby El Centro College was also placed on lockdown during the day.

The shooting happened a block from where the July 7, 2016, ambush occurred during which four Dallas police officers and a Dallas Area Rapid Transit officer were fatally shot.

In that attack, the gunman entered El Centro and fired from a window at the fifth officer who was killed. The shooter, Micah Johnson, was killed with explosives delivered by a police robot.

Dallas police detonated a suspicious device about 10:40 a.m. that was found in the 2003 Nissan Altima Clyde had driven to the courthouse. The blast was strong enough to shake sapling trees blocks away.

DeSarno would not confirm Monday afternoon whether law enforcement found explosives inside the car.

Police also checked downtown for other suspicious devices, and many buildings downtown were locked down or evacuated.

The investigation

DeSarno said more than 200 FBI agents and other law enforcement personnel are investigating the shooting. Investigators expect they will have multiple videos from several angles to watch as they determine what happened.

The Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing Clyde's assault weapon, DeSarno said. Clyde had more than five 30-round magazines, he said.

The FBI searched Clyde's apartment in Fort Worth but did not provide details about what was found.

Joycelyn Mendoza, who lives in the apartment directly above Clyde's, said FBI agents removed a large black box from the complex. The FBI also questioned her about what she knew about Clyde, she said.

"I told them honestly, it smells like marijuana around there most of the time," she said.

Other people seemed to live in Clyde's apartment or visit frequently, she said. A woman from his apartment sometimes complained when Mendoza's 2-year-old son ran around upstairs. Mendoza said she last saw the woman two weeks ago.

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She said the FBI showed her pictures of two men, only one of whom she recognized.

'I just ran'

Ed Modla was working from home Monday morning at SoCo Urban Lofts, near the federal building, when he heard at least 10 gunshots. He looked outside and saw the gunman running.

"As soon as I saw the shooter, I got the hell away from the window," he said.

He took another peek from his third-floor window a few moments later and saw officers "zeroing in" on Clyde across the street, he said.

Dallas police evacuated the apartment building about 10 a.m., going door-to-door to make sure everyone got out.

Erin Nealy Cox, center, U.S. Attorney for the Northern District of Texas, addresses the media ,along with FBI Special Agent in Charge Matt DeSarno, second right, after a man was shot and killed after shooting at the federal courthouse in Dallas. (Shaban Athuman / Staff Photographer)

Don Miles heard 10 to 15 shots as he walked up to the Commerce Street entrance of the federal building for a 9 a.m. appointment.

"I just ran," Miles said.

Herman Turner, 50, had taken the day off work to run errands. He was on his way to get a cashier's check when he saw Clyde race from the courthouse door near Main and Griffin streets, plant himself in the middle of the street and begin firing his rifle back at the building.

Prior courthouse shootings

Monday's shooting wasn't the first near a North Texas courthouse.

In January 2013, a masked gunman fatally shot Kaufman County prosecutor Mark Hasse in a courthouse parking lot in Kaufman.

Over Easter weekend of that year, the gunman, Eric Williams, later killed Kaufman County District Attorney Mike McLelland and his wife, Cynthia, in their home.

Williams remains on death row. He killed the district attorney and prosecutor in a revenge plot for prosecuting him for stealing county property. Williams' wife, Kim, testified against him and is in prison for her role in the murders.

Law enforcement officers secure the scene after a man fired shots at the Earle Cabell federal building and courthouse in downtown Dallas on Monday morning, June 17, 2019. A revolving door at the main entrance along Commerce Street was shattered. (Tom Fox / Staff photographer)

In January 1993, Hai Van Huynh opened fire in the hallway of the George L. Allen Sr. Courts Building in Dallas, fatally shooting his wife, Ly Dang, and wounding a bystander. He later died from a self-inflicted gunshot wound.

On July 1, 1992, George Lott opened fire inside the Tarrant County Courthouse, killing two lawyers, seriously wounding two appellate judges and grazing another attorney. He died by lethal injection two years later.

Lott had been indicted a few months earlier on aggravated sexual assault charges stemming from allegations that he had sexually abused his son at a motel in Peoria, Ill., according to police and court records. He was scheduled to be tried July 24 on the more serious of the charges, said prosecutor Jim Owens in Peoria, where the son lived.

Staff writers Sarah Sarder, Eva-Marie Ayala and David Tarrant contributed to this report.