Six people were found dead in a suite at the Grand Hyatt Erawan Bangkok.
Initial reports of a shooting were dismissed, with authorities now investigating possible poisoning.
The victims, all Vietnamese nationals, included some with dual American citizenship.
Investigators suspect the deaths were due to foul play, not suicide, and that the bodies had been there for 24 hours. Suspicious substances were found in drinks ordered to the suite.
The US State Department is monitoring the situation.
Prime Minister Srettha Thavisin expressed conce over the potential impact on Thailand's image and tourism, which is crucial to the economy but has not fully recovered from the pandemic.
The country recently expanded its visa-free entry scheme to 93 countries and territories to attract more tourists.
(colombotimes.lk)