Royal Thai Police are probing a report which claimed a senior police officer in Nakhon Sawan accidentally killed a drug suspect in a botched attempt to extort him out of 2 million baht.
An investigation was launched after the claim was made public by celebrity lawyer Decha Kittiwittayanan on Facebook.
According to Mr Decha's post, the case began with the arrest of a man and woman over drug offences by a team from the provincial police.
The officers, he said, pressed the pair into agreeing to pay 1 million baht to be released.
However, a senior officer, a police colonel, demanded the pair double the amount.
As they negotiated the amount, the officer put a plastic bag over the man's head to intimidate him, but he suffocated and died instead, the post said.
Before sending his body to the hospital, the senior officer instructed his subordinates to ensure the man's cause of death was listed as a drug overdose.
He then released the woman from police custody, in exchange for keeping her mouth shut.
A subordinate who witnessed the entire thing decided to lodge a complaint, Mr Decha said.
In response to the complaint, police inspector-general Visanu Prasattongosoth yesterday said national police chief Suwat Jangyodsuk has ordered the investigation be sped up as it has dented the police's credibility.
Pol Gen Visanu said inspector Pol Lt Gen Chettha Komolwanna has been asked to oversee the investigation.
Provincial Police Region (PPR) 6 chief, Pol Lt Gen Apichart Sirisit, has also instructed the Muang Nakhon Sawan police superintendent to assist the investigation, he said.
Pol Gen Visanu added the Nakhon Sawan Police chief was also instructed to join.
If the allegation is confirmed, criminal charges will follow, he added.