Police are looking into whether a group of wealthy business owners from China are behind the growing number of unauthorised night entertainment venues and illegal gambling dens in Bangkok's Ratchada and Sutthisan areas, as well as Pattaya City in Chon Buri.

The investigation followed a raid on a karaoke parlour masquerading as a car wash on Charoen Rat Road in Sathon district early on Wednesday, which led to the arrest of 268 people, many of whom were Chinese citizens.

All those arrested are being investigated by the police.

On the same day, authorities said they have found traces of amphetamine in the system of You Si Hua, a 32-year-old Chinese tourist who died after visiting a pub on Ratchada Road on Sept 17.

After it came to her family's attention that she had passed away "for reasons unknown", her relatives contacted the Chinese embassy in Thailand to find out what happened.

Police said on Wednesday that they believe both the pub on Ratchada Road and the karaoke parlour in Sathon are run by the same people.

Police are also trying to work out where the drugs found at the Sathon karaoke parlour came from.

Not only did the bar offer drugs to its patrons, the staff even offered to keep leftover drugs for their clients' next visit.

Police are also aiming to find out, with the help of Chinese authorities, how these Chinese businesspeople managed to leave their country while strict Covid-19 restrictions are in place.