A BART spokesperson said a custodian was cut on their left shoulder by an individual who was part of a larger group that caused a disturbance.
The incident happened at around 7:45 p.m. at the 24th Street and Mission BART station.
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The spokesperson said the weapon used is believed to be a pocketknife and has not been recovered.
The employee was treated at a local hospital and has been released.
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A juvenile was detained, but is not believed to be the attacker, BART said. That person was eventually released to a parent.
Police are investigating and are looking at surveillance video, which will not be released to the public since it is an active investigation.