Deputy national police chief Pol Lt Gen Winai Thongsong said police had sufficient evidence, including the image of Vivat Yodprasit, to issue an arrest warrant which eventually led to his capture.
His lawyer, Puangthip Boonsanong, on Tuesday accused the police of forcing a confession from her client after visiting him at Bangkok Remand Prison. She said Mr Vivat told her that he was beaten by police.
Ms Puangthip has been assigned by the Network of Students and People for Reform of Thailand to fight the case for Mr Vivat.
Pol Lt Gen Winai, who leads the case investigation, argued that no force had been used against the suspect as the evidence had already cornered him.
The same position was echoed by Tarit Pengdith, the secretary-general of the Centre for the Administration of Peace and Order. He said authorities did not attack the man and that he was not a scapegoat.
The 24-year-old native of Phitsanulok was arrested in Surat Thani on March 19 and taken for a press conference in Bangkok a day later.
He admitted to being hired as a People's Democratic Reform Committee security guard for a daily wage of 300 baht and was the one who shot an M16 assault rifle hidden in a popcorn sack at the red shirts on Feb 1.