A man cursing and muttering incoherently has opened fire aboard a packed Greyhound bus in southern California, killing one passenger and wounding five in a seemingly random attack, authorities and a witness said.

The driver of the Los Angeles-to-San Francisco bus pulled off the highway and the killer got out but was quickly taken into custody without incident, California Highway Patrol Sergeant Brian Pennings said.

The motive was not immediately known, and there was no indication the gunman knew any of the victims, Sgt Pennings said. He addded that several ammunition magazines were discovered along with the gun, which was left behind on the bus.

Investigators outside a Greyhound bus after a passenger was killed on board in Lebec, California
Investigators near the Greyhound bus in Lebec (Jayne Kamin-Oncea/AP)

The bus was travelling on Interstate 5 near the small mountain community of Lebec, about 70 miles north west of Los Angeles, when the passenger started shooting with a semi-automatic handgun. Forty-three people were aboard, including six and eight-year-old children, who were not hurt.

The dead passenger was identified as a 51-year-old woman from Colombia. Two victims were taken to hospital with serious injuries, Sgt Pennings said.

Passenger Mark Grabban, 29, told NBC News the gunman was talking “incoherently” when he boarded the bus in Los Angeles and had argued with someone who had asked him to be quiet.

“He was muttering things, about ‘Wait till we get to the station’,” Mr Grabban said.

He said he heard the gunman — a tall, massive figure, he recalled — curse and cock his gun, firing eight or nine shots. The driver initially thought the gunshots were the sound of an engine problem, the passenger said.

“I dove to the floor right to under the seat. I got to my girlfriend, tried to put her head down,” Mr Grabban said, adding that the gunman aimed towards the back of the bus where he was sitting. “I was just waiting for the next shot. I was assuming I was going to get shot.”

Investigators at the scene
Investigators at the scene (Jayne Kamin-Oncea/AP)

A casing ended up on his lap, he said.

“I saw the blood on the floor of the aisle,” Mr Grabban added. “I looked to the woman on the left, and she wasn’t responding, wasn’t moving or anything. She was lifeless.”

Sgt Pennings said the driver immediately pulled to the shoulder and somehow managed to “coerce the suspect off of the bus”, but Mr Grabban told the Associated Press the gunman panicked and fled the bus by himself, and the driver did not immediately pull away, although passengers were shouting for him to leave.

The driver continued on to the next exit and pulled into a petrol station. “His actions were very appropriate and one could say heroic,” Sgt Pennings said.

Highway patrol officers found the suspect a few minutes later. No immediate charges were brought against the the man, whose name was not released.

Authorities are looking into whether a video camera aboard the vehicle captured the attack, which happened when the bus was at the top of Tejon Pass. It rises to an elevation of more than 4,100ft in Lebec.

Greyhound prohibits passengers from taking guns, explosives or dangerous chemicals on its buses or in their luggage, according to its website. The company declined to answer additional questions about security on its buses.