A gunman who opened fire with a shotgun at a federal building Monday, killing a court security guard and wounding a U.S. marshal before he was shot to death, was upset over losing a lawsuit over his Social Security benefits, law enforcement officials told The Associated Press.

The two officials, who spoke on condition of anonymity because they were not authorized to discuss the case, said 66-year-old Johnny Lee Wicks opened fire with a shotgun at a security checkpoint, touching off a running gunbattle with deputy U.S. marshals.

Although the investigation is continuing, the officials said evidence points to Wick's anger over his benefits case as the motive for the shooting.

Court records show Wicks sued the Social Security Administration in 2008, alleging he was the victim of racial discrimination because his benefits were reduced when he moved from California to Nevada in January of that year. The case was thrown out and formally closed last Sept. 9.

Gunfire erupted at the courthouse moments after 8 a.m., at the start of the work week, and lasted for several minutes. Shots echoed around tall buildings in the area, more than a mile north of the Las Vegas Strip. An Associated Press reporter on the eighth floor of a high-rise within sight of the federal building heard a sustained barrage of gunfire.

A passer-by said he counted at least 40 shots.

"The first shot that I heard was a shotgun blast. I knew it wasn't fireworks," said Ray Freres, 59, a sandwich shop manager and Vietnam veteran who said he was behind the federal court building at the time.

"I heard an exchange of gunfire. I was watching the street," Freres told the AP. "If they were coming my way, I was going the other way."

The U.S. Marshals Service said the victims included a 48-year-old deputy U.S. marshal who was hospitalized and a 65-year-old court security officer who died.

The dead guard was Stanley Cooper, a retired Las Vegas police officer employed by Akal Security, said Jeff Carter, spokesman for the Marshals Service in Washington.

Las Vegas police did not immediately provide information about Cooper. Carter said he was a police officer for 26 years and became a federal court security officer in Las Vegas in 1994.

Authorities did not immediately release the names of the wounded marshal.

U.S. Sen. John Ensign, R-Nev., told reporters it appeared the gunman acted alone and the shooting was not a terrorist act.

In a handwritten lawsuit filed in March 2008, Wicks complained that his Social Security benefits were cut following his move and accused federal workers of discrimination because he is black.

"This case from the start was about race," Wicks wrote in the seven-page complaint, which has occasional spelling and grammatical errors.

"Lots of state worker(s) and agencies have took part in this scam mainly for old blacks who are not well educated," he wrote.

Wicks claimed the benefits reduction actually began in the state of California, after he had a stroke and wasn't able to go to government offices to protest an earlier benefit reduction.

He also claimed that Social Security staff called his new landlord in Las Vegas and told her not to help him.

"I didn't see it or hear it but I know it happen(ed)," Wicks wrote.

The case was formally dismissed Sept. 9 by U.S. District Court Judge Philip Pro in Las Vegas following a hearing Aug. 17 before federal Magistrate Judge George Foley Jr.

FBI Special Agent Joseph Dickey said the gunman, dressed in black pants, shirt and jacket, opened fire in front of a set of security metal detectors just inside the rotunda of the federal building,

"From what witness accounts have said, he walked in with a shotgun underneath his jacket and opened fire when he opened the doors," Dickey said. "Seven officers responded and returned fire."

Ensign said the guard who died had been shot in the chest.

A YouTube video recorded the sound of the running firefight as the man retreated across Las Vegas Boulevard toward another federal building and a historic school.

"I could see guards and everything coming out, and then all of a sudden I just started hearing pop, pop, pop. I mean, just like 30 or 40 shots," said Troy Saccal, a tax services manager who was arriving for work at the time.

Saccal said he thought he saw one guard slump to the ground and another move to help him.

The gunman died moments later in the bushes outside the restored Fifth Street School, where his body remained for several hours.

A Las Vegas police spokeswoman, Officer Barbara Morgan, said the man had been shot in the head.

John Clark, director of the U.S. Marshals Service in Washington, called the security officers heroes.

"The brave and immediate actions of these two individuals saved lives by stopping the threat of a reckless and callous gunman," Clark said in a statement.

Bullet holes marked the entrance of the eight-story modern federal building, which was locked down after the shootout and closed for the day. After police arrived, paramedics helped two people out and down a ramp to ambulances.

A helicopter view showed heavily armed officers in flak jackets scouring the federal building's roof. Shortly afterward, employees in small groups were escorted by armed officers to the auditorium of the Las Vegas Academy, a school three blocks away.

Dickey called the building evacuation "standard procedure" in such an incident.

The gunfire erupted as downtown was busy with office workers and jurors reporting for duty, both at the federal building and the 16-story Regional Justice Center, which houses state and local courts two blocks away.

The state courthouse was evacuated as a precaution and closed for the day, court spokesman Michael Sommermeyer said. Sommermeyer said later he could find no criminal or civil court filings under Wicks' name.

The Lloyd D. George U.S. Courthouse and Federal Building opened in 2002 and is named for a longtime senior federal judge who still hears cases. It has federal courts covering Nevada and offices for federal officials including Ensign and fellow U.S. Sen. Harry Reid. Neither was in the building at the time, authorities said.

The structure was touted as the first federal building built to comply with blast resistance requirements following the 1995 bombing of the Alfred P. Murrah Federal Building in downtown Oklahoma City.

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Barrett reported from Washington. Associated Press Writer Oskar Garcia in Las Vegas contributed to this report.