Suriyan "Mor Yong" Sucharitpolwong died from bloodstream infection at the Corrections Department hospital late on Saturday, Justice Minister Paiboon Koomchaya said on Monday.
The key lese majeste suspect died at around 10pm on Nov 7 and his relatives have already been informed, Gen Paiboon said.
Vittaya Suriyawong, director-general of the Corrections Department, issued a statement about the circumstances of Suriyan's death on Monday.
The statement said at 9pm on Saturday guards on duty at the temporary prison at the 11th Military Circle in Bangkok where Suriyan was detained found him lying in his cell with laboured breathing.
Since he did not respond to their call, the guards alerted the military medical unit, which found him unconscious with a faint pulse.
By the time they took him to the Corrections Department hospital in Chatuchak district at 10.20pm, he did not respond and his pulse could not be detected. His pupils dilated by 4mm and did not respond to light.
The hospital's emergency team tried to apply advanced cardio pulmonary resuscitation (CPR) for over an hour before the duty doctor pronounced him dead, the statement said.
The Institute of Forensic Medicine under the Police General Hospital, having performed an autopsy on him, established on Sunday the cause of death was "respiratory and blood circulation failures due to bloodstream infection", the statement said.
After the death, a fact-finding committee will be set up and report to the department while Pracha Chuen district police will investigate into the death.
About Suriyan's earlier conditions, Mr Vittaya said he had high fever, coughed and was agitated on Thursday and Friday. The military medical unit gave him some pills and told him to rest before the guards found him unconscious.
"Doctors said some air-borne viruses could cause an acute respiratory failure and high-risk patients are the elderly, diabetic people and persons on steroid.
"In Suriyan's case, it's possible his immune system was weak as indicated by the Oct 22 x-ray results, which showed he had a fatty liver," Mr Vittaya explained.
Blood tests during the resuscitation also showed high levels of liver enzyme and abnormally low blood-platelet count, at 60,000 platelets per cubic millimetre, compared to 140,000 to 400,000 in normal people, he added.
"Our assumption is he was infected by a germ that caused bloodstream infection and this led to the respiratory failure and quick death," Mr Vittaya said.
Suriyan, a well-known fortune-teller who helped organise and promote the Bike for Mom and Bike for Dad events, was one of the three suspects arrested in mid-October.
The others were Jirawong Watthanathewasilp, his adviser, and Pol Maj Prakrom Warunprapa, a computer-crime police officer.
Acting on the National Council for Peace and Order's complaint, police arrested the trio on Oct 21 for allegedly extorting money from businessmen by citing the monarchy. They were later charged with lese majeste under Section 112 of the Criminal Code.
Since lese majeste is a national-security offence, the case comes under the jurisdiction of the military court. The three suspects were detained at the temporary prison at the 11th Military Circle in Bangkok.
Pol Maj Prakrom was found hanging in his cell with a shirt around his neck on Oct 24 and died shortly after at the Corrections Department's hospital. Authorities established the cause as suicide.
Gen Paiboon said shortly after Pol Maj Prakrom's death that Suriyan, who had high blood pressure, had been admitted to the department's hospital. He was sent back to his cell two days later after he recovered.
Suriyan was last seen in public on Oct 21 when he was brought to the military court for the first round of detention.
He was not taken to the court when police sought the second detention on Oct 30. The Corrections Department's statement said he had headache, abdominal pains, flatulence, dizziness and slow reaction. Officials said he returned to his cell a day later.