A "flash mob" robbery involving somewhere between 30 and 60 teenagers at Oakland's Coliseum BART station that we briefly mentioned Monday is causing BART to have answer questions about how much of the incident was caught by surveillance cameras, and where BART police were at the time.

As KQED reports, the incident occurred at 9:26 p.m. Saturday when the swarm of dozens of teens jumped fare gates, ran to the second-story platform, and boarded multiple cars of a Dublin-bound train that was stopped there. No weapons have been supported, but the kids proceeded to commit multiple strong-arm robberies, making off with cellphones, purses, and one duffle bag. Two people were beaten badly enough that they required treatment by paramedics for facial injuries, according to KQED.

27-year-old Justine Lin tells the station that she was on the train and at first just heard a lot of banging on doors and commotion, thinking the kids were just "horsing around." But then she says they boarded the train and people were screaming. "They started hitting passengers and stealing their stuff,” Lin said. “They were really attacking one family of three. … They were hitting the dad a lot on his neck and on his head, they were trying to grab his phone. They hit the mother — they stole her purse and her phone." Lin also had her phone stolen in the incident.

The entire attack lasted about a minute and all the suspects fled into the surrounding neighborhood. In total, seven passengers were robbed.

BART spokesperson Alicia Trost says that this isn't the first time that a group of teenagers has run onto a train and committed this type of robbery, but it's unusual because of the size of the group. Trost tells NBC Bay Area, "To have it happen all at once, obviously it was a coordinated [incident]. They knew why they were coming to the station."

As the East Bay Times notes, only six of the train's nine cars had working surveillance cameras at the time, but the station also is equipped with cameras, and BART is reviewing the footage and will be seeking help identifying suspects from the Oakland Housing Authority and police at the Oakland Unified School District. Trost says the footage will not be released publicly because the individuals are all believed to be juveniles.

"It is something that occurs frequently enough that we know it’s a problem, and we are working with law enforcement," Trost tells NBC Bay Area.

Meanwhile Rebecca Saltzman, president of BART's Board of Directors, tells the Chronicle, "We’re taking this seriously. Things like this have happened elsewhere but not at BART.” She may be referring to the large-scale swarm attack by teenagers at a carnival in East Oakland just last month, which involved vandalism and the theft of prizes from booths.

Two BART police officers were apparently at Coliseum Station at the time of the incident, as the East Bay Times reports, but they were in a back parking lot at the time conducting a "robbery suppression patrol." They responded to the concourse as soon as they got the call.

Below, via KRON 4, you can hear audio of the train operator calling in the incident just after it happened. The train was reportedly held in the station for 15 minutes as police investigated. Trost confirmed that BART police presence was increased, using overtime, on Saturday night and that that would continue.

Previously: Day Around The Bay: Gang Of Deplorable Teens Becomes Robbery Flash Mob At Coliseum BART