The government is responsible for bringing all defendants in the Tak Bai massacre case, including a former ruling party MP, to court before the 20-year statute of limitations expires next week, said Natthaphong Ruengpanyawut, the People's Party leader and opposition leader.
Mr Natthaphong made the comments during an interview about the resignation of Gen Pisal Wattanawongkiri from Pheu Thai.
He said the government has to ensure Gen Pisal faces trial no matter whether he still has MP status with Pheu Thai or not.
Gen Pisal is believed to be abroad.
The Tak Bai incident occurred on Oct 25, 2004, when 85 people were killed.
Gen Pisal was commander of the army in the southern region at that time. The case's statute will expire next Friday.
On Tuesday, all seven defendants with arrest warrants, including Gen Pisal, failed to show up at the Narathiwat provincial court to acknowledge charges.
"The government has to bring justice to Thais as this case has created deep wounds for the people of the three southern border provinces for a long time," said Mr Natthaphong.
He also referred to Gen Pisal's resignation letter, which said that he would clarify everything once his health improves, which should be after the statute of limitations expires.
"People will believe this is an attempt to avoid justice if it transpires in this manner. People's trust in the government will decline even further. There is still time for the government to carry out its responsibilities fully," Mr Natthaphong said.
"For our part, we'll keep looking into this and thoroughly uncover the truth."
Former Democrat MP Thepthai Senpong said the Pheu Thai Party did not honestly care about the Tak Bai case because it allowed Gen Pisal to be its party list-MP.
Mr Thepthai said the party did not attempt to have Gen Pisal face justice but allowed him to go on leave for medical treatment abroad.
He said Pheu Thai did not want to lose an MP's seat, so instead of expelling Gen Pisal, it allowed him to resign, allowing another MP on its party list to replace him.
"The Pheu Thai Party cannot deny responsibility for the 85 deaths in Tak Bai. The tragic incident occurred during the government of Thaksin Shinawatra, and its statute of limitations will expire during the government led by his daughter, Paetongtarn," he said.
"If no one is prosecuted, it will create a culture in which people who commit a crime can get away with it."
Kamolsak Leewamoh, a Prachachat Party MP for Narathiwat, demanded the state arrest and prosecute all defendants in the Tak Bai case.
"We demand justice for the 85 people who died, the 51 who were disabled, and the over 1,280 people who were detained. The Prachachat Party calls for the defendants to go through the legal system to reveal the truth," he said.
