Story highlights Witness tell CNN affiliates a prisoner was on the witness stand, testifying about gang life

Woman says man who was shot appeared furious

Officials: Defendant -- alleged to be a gang member -- rushed at witness on first day of trial

Defendant charged with racketeering conspiracy, accused of several robberies

A U.S. marshal fatally shot a defendant in a Utah federal courtroom Monday morning after he rushed at a witness as his trial began, officials said.

Siale Angilau, an alleged gang member charged with racketeering conspiracy, was shot several times at a Salt Lake City federal courthouse after Angilau attacked someone who was on the witness stand, Judge Tena Campbell said in a court document about the incident.

He later died at a hospital after being shot in the chest, the FBI said in a news release.

"There were people yelling at him, telling him to stop, and he just didn't stop," Sara Josephson, who was in the courtroom, told CNN affiliate KSTU . "He kept going forward with his furiousness."

The station quoted FBI Special Agent in Charge Mark Dressen, who said Angilau grabbed a pen and charged a witness.

"I believe he was shot in the chest multiple times," Dressen told KSTU.

People were ducking behind benches, Perry Caldwell told the station. He said one person yelled, "Get on the floor!"

"He (Angilau) was falling as (the marshal) was still shooting him," Caldwell said.

Witnesses told CNN affiliates that the witness was a prisoner who was testifying about his life in a gang.

Angilau was the only person shot, said Melodie Rydalch, spokeswoman for the U.S. attorney for Utah.

Campbell declared a mistrial, noting the shooting happened in front of jury members, who were "visibly shaken and upset."

"The court finds that this occurrence in the courtroom would so prejudice Mr. Angilau as to deprive him of a fair trial," Campbell wrote.

Angilau was one of nine alleged members of the Tongan Crip Gang charged in a 2010 indictment with racketeering conspiracy. The indictment alleges Angilau committed several convenience store robberies in Salt Lake City and assaulted the stores' clerks from December 2002 to July 2007.

Angilau was charged with several other crimes, including assaulting a federal officer.