A mentally deranged man who opened fire inside a crowded California bus was wrestled to the ground and disarmed by fellow passengers, but not before his rampage left one man dead and five others wounded, two of them in critical condition.

It was the second random shooting spree in the US in less than 24 hours.

Across the country in Texas, a hail of bullets in a residence hall at Agriculture and Military University left two women dead and an infant in hospital.

The driver of the Los Angeles-to-San Francisco Greyhound bus pulled onto the shoulder, where some of those aboard dragged the killer off the vehicle and held him until police arrived, California Highway Patrol Sergeant Brian Pennings said.

Other passengers performed first aid on the wounded.

Anthony Devonte Williams, 33, of Capitol Heights, Maryland, was arrested on suspicion of murder and attempted murder.

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The motive for the shooting was not immediately known, nor was there any indication the gunman knew any of the victims or that they had provoked him.

The bus was only minutes into its journey  when the passenger started shooting with a semi-automatic handgun shortly before 1.30am.

Forty-three people were aboard, including children, who were not hurt.

The dead passenger was identified as a 51-year-old woman from Colombia.

A 45-year-old man and a 19-year-old woman were in critical condition in hospital, a 39-year-old woman had major injuries, a 49-year-old man had moderate injuries and a 50-year-old woman had minor injuries.

Passenger Mark Grabban, 29, said the gunman had been sitting with his leg sticking out into the aisle, muttering to himself incoherently.

“He was just saying weird stuff like, ‘You don’t know me like that’, ‘Wait ’til we get to the station’, ‘Get away from the dude in the striped shirt’,” Grabban said in a series of Instagram messages.

Suddenly, the man started swearing and shooting, Grabban said.

Pennings called the actions of the driver and the passengers who disarmed the gunman heroic but did not provide immediate details.

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In the Texas incident, the two female victims and a wounded toddler were found after an anonymous call to police.

The university campus was locked down until police, who have shared no details about the incident, declared there was no threat — an announcement taken as confirmation that the shooting likely a murder/suicide.

The wounded two-year-old was rushed to hospital where his condition was listed as stable.

-with AAP