A gunman in a black trench coat opened fire Monday morning in the lobby of the Federal Courthouse in downtown Las Vegas, killing a court security officer and wounding a deputy United States marshal before fleeing. He was then shot in the head and killed nearby.

“The suspect was in the lobby, but he never made it past the security checkpoint,” said Barbara Morgan, a spokeswoman for the Las Vegas police.

The building is a huge structure that houses federal courts as well as other agencies, including the offices of Senators Harry Reid, the Democratic majority leader, and John Ensign, a Republican. Both Senators were in Nevada, their offices said, but not in the building.

The gunman entered the building shortly after it opened at 8 a.m. and began firing in the lobby. An Associated Press reporter within sight of the building counted 20 shots over several minutes, although it was not clear how many were fired by the gunman and how many by responding law enforcement officials.

Police and federal agents poured through the building looking for signs of any other suspects, but none were found.

Two hours after the shooting, the complex was still being evacuated floor by floor, with groups of about 20 people being escorted several blocks away while surrounded by several officers toting large rifles. The courthouse was blocked off for at least a five-block perimeter.

Paramedics rushed at least two people down a ramp to waiting ambulances, where they were taken to the trauma unit of the University Medical Center. Federal officials confirmed that the security officer, who was 65, had died. Roxana Lea Irwin, chief deputy United States marshal for the Nevada district, said she saw shotgun casings on the lobby floor, an indication of the kind of weapon that was used.

There was no immediate word on the identity of the suspect, or whether the shooting was a random act of violence, a vendetta or something else. There was also no initial indication that terrorism was involved.

Joseph Dickey, a special agent with the Federal Bureau of Investigation, said the gunman died shortly after the shootout.

The building, the Lloyd D. George Federal Courthouse, at 333 Las Vegas Boulevard South, is blocks from several casinos, including Fitzgeralds Casino and Hotel and the Plaza Hotel and Casino. It is also close to the University Medical Center and the Valley Hospital Center.

Senator Reid’s office said that his staff was fine. “My thoughts are with the victims of today’s shooting and their families,” Senator Reid said in a statement. “The law enforcement personnel who protect the courthouse put their lives at risk every day to keep the people who are inside safe, and I greatly appreciate their service.”

Asked if the senator thought the incident was related to the presence of two senators in the buildings, Jennifer Cooper, Mr. Ensign’s spokesman, said the office was still learning details of the shooting.

The courthouse — with its giant articulated column and circular atrium entrance shaded by a canopy in line with the roof — is architecturally significant, having received a citation for design excellence from the American Institute of Architects’ Committee on Architecture for Justice.

“The design makes a bold statement in the scale of the canopy, monumental column and stairs at the building entry,” the committee said. “The scale and detailing of these elements serve to establish a signature identity for the courthouse.”

Steve Friess contributed reporting from Las Vegas.