San Francisco police raided what appeared to be an illegal casino operation inside of one or more Tenderloin storefronts Tuesday evening, seizing 16 gambling machines.Apparently, it was fairly well known to people in the neighborhood that an underground casino was operating out of the 200 block of Leavenworth Street for at least the last year. As the Chronicle reports, the SFPD obtained a warrant and raided three different businesses on that block Tuesday evening: T.L. Market & Deli, SF Discount Market, and Ryan’s Laundry.Police were seen removing electronic slot machines and other gambling machines from T&L Market, and it appears that the machines may have been spread between the three business. A total of 16 machines were seized, and the Chronicle saw officers prying open cash boxes inside the machines.The owner of a market at the other end of the block, Bora Peang of Empire Market, told the Chronicle that she had heard her customers talking about winning or losing money while gambling nearby.The police department hasn't put out any official release or information about the raid, and it's not clear whether there was one or multiple business owners involved.The Chronicle obvserved four people being led out of one of the businesses in handcuffs. The total number of arrests is also not known, but charges may be revealed in the coming days.Illegal gambling dens like these crop up every so often, but we haven't heard about one in the Tenderloin for quite a while. Nine years ago, a former specialty soda shop on Mission Street became an underground casino until it was busted. And a year prior to that, in 2014, an illegal gambling den was shut down in the Excelsior.Anyone with information on this case is asked to contact SFPD at 415-575-4444 or text a tip to TIP411.Photo via Google Street View