The man fired several shots at 11:46 a.m., although none appeared to have been directed at the White House, according to the Secret Service. He then dropped to the ground as people fled the area.
“Guns again!” Melaku-Bello heard someone shout. “Guns again!”
One video posted on Twitter showed people sprinting from the area as sirens blared in the background and emergency vehicles sped toward the scene.
“Do you think that was shots?” one woman asked, before she began to sob.
No one else was injured, the Secret Service said. D.C. police, who will lead the investigation, announced just after 2 p.m. that the man had died and that officers were working to notify his family.
President Trump and the first lady were at his Mar-a-Lago resort in Florida at the time of the shooting. The president was briefed on the shooting.
Early Saturday afternoon, Secret Service personnel searched and photographed a maroon Honda sedan on K Street, two blocks north of Lafayette Square, which is opposite the north side of the White House. From the trunk, they removed a clear plastic crate, a large blue bag and what appeared to be a cooler, which investigators carefully examined.
On the sidewalk were other items apparently taken from the car, including a pair of neon-orange running shoes.
Stephanie McCrummen, Lynda Robinson, Philip Rucker and Carol Leonnig contributed to this report.