The alleged gunman in the killing of Sutin Tharatin, a protest leader of the anti-government People's Army and Energy Reform Network, has died of a suspected asthma attack while in detention.
Surakit Chaimongkol was unable to breathe on Thursday afternoon and was rushed to the Correctional Hospital, but doctors failed to revive him, Corrections Department chief Witthaya Suriyawong said yesterday.
Mr Witthaya learned of the death from Ayut Sinthop, the head of Bangkok Remand Prison, where Surakit was held.
Mr Witthaya insisted the detainee was not assaulted, but he will wait for a formal court ruling on the exact cause of death, following an autopsy conducted by the Institute of Forensic Medicine.
An initial investigation found Surakit had diabetes and asthma, but Thursday was the first time he had experienced breathing difficulties in prison, Mr Ayut said.
Surakit was arrested on July 8 at his house in Samut Prakan's Phra Pradaeng district after police obtained a warrant for his arrest.
Witnesses who were wounded during the attack on Sutin on Jan 26 earlier told police they saw the 36-year-old suspect nearby.
Sutin was shot dead as he staged a demonstration against advance voting for the Feb 2 election, called by the then Yingluck Shinawatra government after the House dissolution late last year.
The incident occurred as Sutin led protesters to block voting near Wat Sri Iam in the Bang Na area.
Surakit denied involvement in the attack, saying he was nearby because he went to cast a ballot at a polling booth inside the temple.
However, he admitted to owning an 11mm-pistol which was discovered in his house.
After studying security camera footage, police believe Sutin was shot from a low angle and from a distance of no more than 20 metres.