Videotape that might have helped police lay charges against a gang of girls who beat and robbed a teenager at a Vancouver SkyTrain station last week has been lost, CBC News has learned.

"We're all human," Staff-Sgt. Ken Schinkel, a transit police officer, said Tuesday. "People do make mistakes, and unfortunately sometimes the mistakes we make affect people in pretty severe ways."

The security cameras were rolling at Nanaimo Station on Tuesday when a pack of teenaged girls followed Sheshleen Datt to the bottom of the stairs and attacked her around 9:30 p.m.

"They tugged on the back of my purse and they yelled, 'Hey!'" Datt told CBC News Friday. "I turned around and … I get a punch in my face."

Vancouver police arrived at the station and tracked down her missing purse and a knife, Datt said. Police apprehended six girls shortly after.

Police immediately requested the tape, but a transit police officer neglected to pass the request on to SkyTrain's control centre.

The system uses old black-and-white cameras using VHS tape that records over itself every two hours, and that's how the evidence was lost.

SkyTrain is in the process of converting to a $1.8-million digital system that will store data for seven days.

But SkyTrain CEO Doug Kelsey is not convinced the current system's 800 cameras prevent crime, nor that there's a communication problem with police.

"You know what? I think we've got an aberration here, frankly. There's nothing here that would say strategically we've got a problem. If we do, we'll make the corrections."

Transit police confirmed there will be an internal investigation of the incident, but that's cold comfort for Datt, who only began riding the SkyTrain to work again on Tuesday.

"That video would have helped a lot … it would have shown everything. It would have made things a whole lot easier," she said.