BART Police released a mug shot of the suspected stabber, 32-year-old Robert Dolph of San Francisco.
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A stranger stepped-up to stop the violent attack.
"This man that intervened, he is a hero, very courageous. Had he not intervened the two victims quite possibly could have been killed," said Deputy Chief Lance Haight, BART Police Department.
RELATED: Suspect in custody after stabbing at Coliseum BART station in Oakland
As of Sunday, a man in his 50s is in critical condition and his sister, a woman in her 60s, is in fair condition.
They boarded BART in South San Francisco headed to the East Bay. Police say Dolph boarded the train at Civic Center.
"At one point he became somewhat fixated on these two victims and he was saying things directly to them, but it didn't seem to pertain to them it was out of context. They didn't know what he was talking about," said Haight.
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Alarmed by his strange behavior the siblings exited the train at the Coliseum station and tried to get away from him. Police say he blocked the fare gates. The victims went through the gate by the station agent and that's when police say Dolph stabbed them both in the head and the man in the chest with a large fixed blade knife.
BART is not releasing the footage, but the stabbing was captured on camera. At least seven station cameras are within line of sight from where the stabbing took place.
"We have retrieved the video and it's being reviewed and it will absolutely be part of the criminal prosecution," said Haight. He added that they'd like any witnesses who've not already spoken with investigators to come forward.
During the attack the station agent called for help. Additional Good Samaritans helped hold the man down on the ground. A nearby BART officer arrived in less than two minutes.
BART riders we spoke with say they're not afraid to ride BART. They all noted an anecdotal increase in the amount of people on the trains who appear to be transients or suffering from mental illness.
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Riders praised the Good Samaritan who stopped the brutal attack.
"Remember that we do live in a world that is good and there's a lot of good people and people will step up when needed," said Amber Childress, an Oakland resident.
Dolph is in custody at Santa Rita Jail facing attempted murder charges. Detectives say he confessed to the stabbing, but did not give a motive.
The Coliseum station remained closed for hours to investigate the stabbing. BART ran a bus bridge to get Warriors fans to Oracle arena.