Pol Col Vachira Yaothaisong, commander of Highway Police sub-division 2, shot himself dead at his house on Monday, a police source said.
The officer was the supervisor of Pol Maj Siwakorn Saibua, who was murdered by a gunman at a dinner party at the house of Praween Chankhlai, alias Kamnan Nok, in Nakhon Pathom last Wednesday.
Pol Col Vachira, 44, was one of the 25 police officers who also attended the party and are being investigated. He died at his home in a housing estate in Lam Luk Ka district, Pathum Thani.
The reason for his suicide was not immediately known. However, he had been named as the person who called Pol Maj Siwakorn on the phone, telling him to go to Mr Praween's house, where he was later killed by gunman Thananchai Manmak.
Pol Col Vachira was reported to have sought treatment for depression and stress at Phramongkutklao Hospital after the death of Pol Maj Siwakorn.
