The gunman suspected of killing Suthin Taratin, who led a rally against advance voting in January, was arrested on Tuesday at his home in Samut Prakan's Phra Pradaeng district, police said yesterday.
Surakrit Chaimongkol denied the charge of conspiring to commit murder but admitted that he owned the gun that police suspect was used in the crime, police said.
Leaders of the People's Army to Overthrow the Thaksin Regime, and Suthin's mother Saroj, presented flower bouquets to thank Pol General Aek Angsananont, a deputy national police chief, and police in charge of the investigation.
They also brought evidence to show police that Thawatchai Promchan, a core leader of the People's Army to Overthrow the Thaksin Regime, who had been arrested, had nothing to do with Suthin's death.
No progress had been made in Suthin's case in six months under the Yingluck government.
The investigators said they were confident that they had nabbed the gunman after viewing footage from security cameras more than 20 times. They said the killer was 20 metres away.
Surakrit was three metres from Suthin while he was shot.
Police believe the bullet had an upward trajectory, as the bullet hit Suthin in his right chest and passed through his head.
Police checked records and found that the owner of the suspected murder weapon was Surakrit, who belongs to a rival political group.