The bodies of six people were found Tuesday in a luxury hotel in downtown Bangkok and their deaths may have been caused by poisoning, police and officials said.

Bangkok police chief Lt. Gen. Thiti Sangsawang identified the dead as two Vietnamese Americans and four Vietnamese nationals, and said there were three males and three females. Investigators said the bodies were foaming at the mouth, an officer from the Lumpini police station said on condition of anonymity because he wasn’t authorized to release information.

The victims had booked several rooms at the Grand Hyatt Erawan Hotel under seven names, and some were staying on a different floor from the room where they were found dead, Thiti said. Police are still looking for the seventh person included in the booking, Thiti said at a news conference at the hotel.