Police are stepping up the hunt for the second suspect in the murder of a 17-year-old student in a public park in Trang six years ago, in the wake of the controversial execution on Monday of the youth's convicted killer.

The teenager was attacked by two young men on July 17, 2012 and killed. He was stabbed 24 times. They stole his mobile phone and wallet.

Theerasak Longji, 26, was put on trial for the crime and convicted of aggravated robbery. He was put to death by lethal injection on Monday afternoon, Thailand's first legal execution since 2009.

The second, unidentified, suspect fled with the victim's possessions, deputy Muang Trang police chief Prasoet Songsaeng said on Wednesday. He was not the killer.

When the victim tried to run away, Theerasak chased him down and stabbed him to death, Pol Lt Prasoet said. The murder was intended to cover up the crime, he said.

Witnesses identified Theerasak, who was wanted in another five criminal cases, as one of the attackers. He was arrested the following day and his knife seized as evidence, but he denied the charges, Pol Lt Prasoet said.

Investigators in the case returned to the park on Wednesday, six years after the murder, as they planned a renewed hunt for the second suspect.

Theerasak's execution has generated controversy, especially across the internet. Amnesty International, which is against the death penalty for any crime, condemned the execution as deplorable.

Theerasak's mother still believes he was innocent.