Staff at the Imperial Spa in Vichit, near the Sunday market, are also likely to face questioning because everything points to it being an inside job.
The female owner and a receptionist were held up by a man holding a gun and a second man holding a knife about 9.45pm, with 10 Chinese tourists being treated further inside the premises.
Security camera footage shows both bandits wore full-face motorcycle helmets, long-sleeved shirts, trousers and white gloves. They rode off on a Honda Click motorcycle.
''We believe the bandits were tipped off that the money was at the spa to pay staff on Sunday,'' said a Vichit Police Station investigator.