LONG BEACH, Calif.- The shooting at a Southern California federal building of a high-ranking Immigration and Customs Enforcement official was over an unspecified disciplinary matter, a person familiar with the case told The Associated Press Friday.

Authorities haven't elaborated what led to Thursday's shooting at the Glenn M. Anderson Federal Building in Long Beach, but apparently it was over a dispute between the gunman and the wounded man, said the person who requested anonymity because the investigation is ongoing.

A law enforcement official also told the AP that the person wounded is a high-ranking ICE official and the agent who was killed was a subordinate who did not report directly to the man he shot. The official also requested anonymity because of a lack of authorization to release details.

A third agent intervened and shot the gunman to prevent more rounds being fired at the victim, the FBI said.

The gunman died at the scene and the wounded agent was hospitalized at St. Mary Medical Center. ICE Special Agent in Charge Claude Arnold would only say he was stable.

St. Mary's hospital trauma director James Murray told KCAL-TV that the injured agent had multiple gunshot wounds, but he didn't give details. The victim's vital signs were "good for now," Murray said.

The names of the dead gunman, the victim and the agent who fired the final rounds were not released.

The shooting happened just before 6 p.m. Thursday on the seventh floor of the Glenn M. Anderson Federal Building in Long Beach, about 20 miles south of downtown Los Angeles.

It was described by the FBI's Steven Martinez as a case of "workplace violence involving two federal agents in their office space." He offered no other details about what led to the initial shooting

There were conflicting early reports about the number of people shot, with local authorities saying two were dead and one wounded, while ICE said one was dead and one wounded.

The Long Beach federal building houses ICE, the Internal Revenue Service and the U.S. Probation and Parole Office.

"At times like this, words honestly seem inadequate. When something like this happens in our offices, it's incomprehensible," Arnold said.

Along with the FBI, the shooting was being investigated by ICE's Office of Professional Responsibility and Long Beach police.