An investigation is underway of a viral video of a BART police officer using a Taser on a man not once, but twice.

The cellphone footage of the January incident shows BART police confronting and Tasing an apparently drunk passenger aboard a train that was stopped at San Bruno station.  

Riders can be heard saying "but he didn't do anything" during the incident.  

The man was ultimately taken into custody for resisting arrest.

You can view the footage below, but be warned, it contains explicit language: