The woman boarded the 217 Metro bus in Culver City at about 5 p.m., on her way home from her special education school. The 18-year-old with the mental capacity of a 10-year-old had only recently been allowed to start taking the trip on her own.
A stranger boarded behind her.
He followed her to the back of the bus, authorities said, and without warning began raping her.
The attack lasted for 10 minutes Wednesday afternoon as the bus traveled south through Baldwin Hills, making two stops as the rape continued, authorities said. There were several people on the bus when the two boarded, but some exited during the attack, possibly unaware of what was happening at the back of the bus.
The assailant only ended the attack as the bus was reaching its final stop, where he left the bus, authorities said.
The brazen crime reverberated around the sprawling Metro system Friday. Portions of the rape were captured on a surveillance camera and Los Angeles County Sheriff's Department officials arrested a suspect, Kerry Trotter, Friday morning just hours after releasing still photos of the alleged attacker.
Authorities said Trotter, 20, is a parolee and a transient who had previously been investigated on suspicion of sexual assault.
"It was a crime of opportunity," said sheriff's Sgt. Dan Scott. "Unfortunately, [the victim] was in the wrong place at the wrong time. He followed her onto the bus and assaulted her."
Metro officials were quick to note that sexual assaults and other violent crimes are relatively rare on its network of buses and rail lines. Three rapes have been reported this year on a system that recorded millions of commuter trips.
But despite the numbers, passengers said Friday that the rape left them uneasy. In a region where getting around is usually about being in a car alone, bus commuters say mass transit exposes them to all kind of people and situations, both good and bad.
Faydra Caldwell, 23, said every time she rides the bus she instinctively notes what other passengers are wearing in case she might have to report them to the police. It's a habit she has developed since her phone was snatched by another bus rider.
"You don't know what people are capable of doing," said Caldwell, a student at West L.A. College.
Another rider, John Wilson, said that a few months ago he had to shove a man off the bus because the man was patting a woman's head and making sexual remarks. No one else intervened.
"The bus driver was really angry at me," said the 54-year-old church executive. "He said, 'Don't take the law into your own hand.' I said, well, you weren't doing anything and the passengers sure as hell weren't."
In this week's incident, both the suspect and the victim got onto the bus at about 5 p.m. at the corner of La Cienega and Jefferson boulevards, near the new Expo Line rail station.
Sheriff's officials said they doubt the bus driver or the passengers on the bus knew what was happening. Scott said detectives believe Trotter was riding buses looking for potential victims.
"He immediately went to her and began the assault," Scott said. "...The suspect had his back to the front of the bus. People generally think of a rape as some kind of attack where someone's thrown down. Its not always the case."
"The victim did not scream," he added. "The victim told our detective that she was shocked, and didn't know what to do, and was in fear of her safety and her life."
Detectives are seeking riders on the bus, including one passenger they believe may have witnessed the incident, though they can't be sure. He "did not jump up and scream rape or that someone was being assaulted," Scott said.
Trotter has a history of run-ins with the law, according to records. Last year, he was convicted of drug possession. In April, he was convicted of grand theft and sentenced to a year in jail, but did not serve his full sentence. In June he was arrested again, and in September admitted to violating probation. He was released from jail Sept. 28. On Oct. 15 he was arrested again by Redondo Beach police and served 10 days in jail. Records did not include specifics on that offense.
Authorities said Trotter had been investigated on sexual assault allegations before but never charged.
This summer, the Los Angeles County Sheriff's Department created a team of nine deputies who swarm transit lines where crime has spiked. Most of the crimes they've cited were misdemeanors or low-level infractions such as not paying the fare.
Authorities said their greatest asset for stopping rare, serious crimes such as Wednesday's are other commuters.
Sheriff's officials were skeptical that anyone on the bus Wednesday noticed the attack and failed to act, but Sheriff's Capt. Holly Perez acknowledged that sometimes witnesses are afraid to interfere.
"I think that's human nature," she said. Bus drivers are required to dispatch deputies -- or flip a silent alarm -- if an incident occurs on board. Perez said many drivers take ownership of their routes, stubbornly guarding their bus passengers.
But in a brief interview Friday, one bus driver in Culver City said it can be difficult to monitor passengers while keeping an eye on the road.
"We drive. That's what we do," he explained. "We are not the police."
Times staff writer Richard Winton contributed to this report.