Authorities have arrested an apparently mentally disturbed man accused of a brazen shooting spree Friday in which he fatally shot a friend, dumped the corpse in the middle of South Broad Street, shot another man outside the Orleans Parish Criminal District Courthouse and opened fire on a third victim blocks away in what police described as a “road rage incident.”

The suspect, Alvin Richardson, 47, was taken into custody early Saturday in Jefferson Parish after telephoning Gretna police about 8:15 a.m. from a pay phone outside a drugstore and complaining that someone had been following him. Richardson claimed there were “some people outside my hotel room, and they said they were coming in to get me,” said Capt. Russell Lloyd, of the Gretna Police Department.

Responding officers asked Richardson why someone might want to harm him, and he promptly acknowledged he’d been “on the bridge (Friday) when that murder happened in New Orleans,” Lloyd said, referring to the Broad Street overpass on which a 29-year-old man was found fatally shot.

“All he said was he was there when it happened,” Lloyd said. “Basically, he was saying that he didn’t have any involvement in it.”

The dead man, whose name has not been released by the Orleans Parish Coroner’s Office, was said by police to be a friend of Richardson’s who was riding in his pickup when Richardson shot him multiple times and pushed him out of the vehicle.

Gretna police recovered a .40-caliber gun on Richardson’s person and booked him into the Jefferson Parish Correctional Center as a felon in possession of a firearm; he faces two counts of attempted murder and one count of second-degree murder in New Orleans.

Homicide detectives, scrambling to solve a high-profile crime that happened within sight of the New Orleans Police Department headquarters, identified Richardson as the suspected shooter after interviewing a number of witnesses.

The shootings happened near a busy intersection about 2:30 p.m. Friday, and several people, possibly including jurors taking a smoke break during a murder trial, heard gunfire or saw some part of the rapidly unfolding crime scene.

Sgt. Nicholas Gernon, commander of the NOPD’s Homicide Division, said Richardson has a history of mental illness and served a lengthy prison sentence for the attempted murder of a police officer in the late 1980s.

“Attempting to assign a logical explanation to his irrational actions is something we haven’t been able to do,” Gernon said. “At this point, we don’t know why he opened fire on his friend.”

The exact sequence of the shootings remained somewhat murky, and Tyler Gamble, a Police Department spokesman, said detectives were still investigating which victim was shot first.

Witnesses said Richardson had stopped his pickup, a silver 2011 Dodge Ram, on South Broad Street near the courthouse when gunshots rang out. Witnesses reported seeing the passenger door of the vehicle swing open as Richardson drove away, approaching the overpass.

At some point, before dumping the body, Richardson allegedly shot and wounded a Criminal District Court employee who was standing at South Broad and Perdido streets. The employee, 32-year-old Sean Halligan, suffered a graze wound to the abdomen and was treated and released from the hospital Friday.

Halligan had been working Friday in the Section H courtroom of Judge Camille Buras, who was presiding over the jury trial of Thaddeus Ross. Investigators said there is no evidence that Halligan had been specifically targeted because of his profession or the criminal proceedings, which resulted in Ross being convicted of second-degree murder.

After Richardson fled the scene of the first two shootings, Gernon said, he encountered an “individual on a motorbike” at Clio and Lopez streets. Gernon said Richardson and the driver of the motorbike “had a bit of a road rage incident, at which point (Richardson) opened fire on that person as well, striking his motorbike.” That person was not injured, Gernon said.

“We believe that (shooting) was done while he was fleeing the scene,” Gernon said.

While Lloyd, the Gretna police captain, said Richardson initially denied involvement in the shooting, Gernon suggested the man incriminated himself in a later interview with New Orleans police.

Richardson’s own statements led police to the pickup used in the shootings, which the authorities were processing for evidence Saturday evening. Gernon said the vehicle contained “several spent shell casings on the floorboard, as well as a large amount of blood on the front passenger seat.”

The shootings nearly impacted the proceedings in Section H, where Ross, 24, was being tried in a 2011 shooting that killed one and injured three others, including a 3-month-old girl, in an incident that prosecutors referred to as the “Valentine’s Day Massacre.” Ross’ defense lawyer, Morris Reed Jr., said four jurors had been smoking outside the courthouse before closing arguments and witnessed the shootings.

He said Buras, the judge, initially agreed to postpone the case until Monday “to let everything die down,” a decision Reed said was entered into the court record. Reed said the judge changed her mind and allowed the jury to begin deliberating after District Attorney Leon Cannizzaro came to the courtroom to speak with her, apparently persuading her the case should proceed. The jury deliberated for just half an hour before returning a guilty verdict, Reed said.

Reed acknowledged that Cannizzaro’s prosecutors had “argued a good case,” but he said the judge’s decision was unfair to his client.

“I think procedurally, it wasn’t done properly,” said Reed, adding that he moved unsuccessfully for a mistrial and intends to appeal the case. “I was shocked she would push it through like that, especially in such a high-profile moment.”

A spokesman for Cannizzaro said there was no evidence that any jurors actually saw the shootings but that Cannizzaro was concerned they might be influenced by media coverage of the event if deliberations were delayed.

Staff writers John Simerman and Benjamin Oreskes contributed to this report. Follow Jim Mustian on Twitter, @JimMustian.