It happened near this runaway truck ramp on Northbound I-5, north of Fort Tejon.
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The CHP says passengers acted heroically by disarming the suspect and removing him from the bus.
But it's unclear exactly how they intervened.
"The suspect was left on the right shoulder, northbound Interstate 5, north of Fort Tejon, and the driver drove the bus to the very next exit, which is here at Grapevine," says CHP Sgt. Brian Pennings.
The bus driver came to a stop at the Valero gas station on Grapevine Road.
As other passengers performed first aid on the victims, CHP officers found the shooter on the freeway shoulder and took him into custody.
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Officials say his weapon, a 9-millimeter handgun, was left inside the bus.
"In addition to the handgun, I can also tell you that there were also several other magazines that were located with the handgun," Pennings said.
The bus left Los Angeles at 11:30 p.m. on Sunday and was on its way to San Francisco, with scheduled stops in Avenal and Oakland on the way.
Dozens of passengers were on board at the time of the shooting, including two children who were not hurt.
As part of a statement, Greyhound said:
"Our thoughts and prayers go out to everyone and every family member impacted by the incident today."
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The CHP says they don't have anything to show that the suspect knew any of the victims or other passengers.
"There's no evidence to indicate that there was terrorism involved or anything like that," Pennings said. "We are still trying to establish a motive."
After witnesses gave statements to authorities, they were free to leave.
The CHP says some were picked up by family members.
Others continued their trip on Monday afternoon by getting on another Greyhound bus.
The CHP says they also had a bomb dog search the bus, but the dog didn't find anything else inside the bus.