For 10 agonizing minutes, an 18-year-old mentally disabled woman was raped in the back of a near-empty Los Angeles County bus as it rolled past a state park and a cemetery and no one did anything about it.

Several passengers and the driver were apparently unaware that a crime was being committed in the 45-foot-long bus, but one person on board tried to alert the driver and was unsuccessful.

Authorities arrested Kerry Trotter, 20, on Friday thanks to an anonymous tip by someone who saw an image of the suspected attacker taken by a surveillance camera on the bus that was publicly released.

Trotter was booked for investigation of rape and was being held on $1 million bail, authorities said. It wasn't immediately known if he had retained an attorney.

Investigators said given that Trotter was standing up with his back to the front of the bus and the victim was sitting down that other passengers wouldn't know a rape happened.

"People generally think of a rape as some type of an attack where someone is thrown down. It is not always the case," said Sgt. Dan Scott of the Los Angeles County Sheriff's Department. For passengers, it "seems reasonable that they would not know what was going on."

The attack occurred Wednesday during the rush-hour commute as the woman, who was returning from school, and Trotter boarded the bus at a stop in suburban Culver City. The woman went to the back of the bus and was followed by Trotter where the assault occurred, authorities said.

There were only four or five passengers on board during the five-mile leg when the rape happened and by the end there was only one who remained and tried to get the driver's attention. Investigators are still looking for that person.

The woman, who has the mental capacity of a 10-year-old, did not scream for help, but she did tell the driver she had been raped after the man exited the bus, Scott said.

"The victim told our detective that she was shocked and didn't know what to do and was in fear for her safety and for her life," Scott said.

Trotter, who is unemployed, has previous convictions for grand theft and possession of rock cocaine, according to court records. He was previously investigated for sexual assault, but no charges were ever filed, Scott said.

Trotter and his victim do not know one another. Authorities wouldn't say if DNA evidence had been recovered.

"We believe it was a crime of opportunity, that unfortunately she was in the wrong place at the wrong time," Scott said.

It was the third rape so far this year on county buses that annually carry millions of people.

Bus drivers are trained to call transit dispatchers when they become aware of a possible crime, said Marc Littman, a spokesman for the Metropolitan Transportation Authority. If a crime is in progress, there is a silent alarm on board that can be tripped.

"This is an extremely rare occurrence," he said.

It was unclear if the driver saw what transpired. He was being interviewed by authorities who don't suspect he did anything improper.

"We don't believe anybody else did anything wrong," Scott said.