The DNA of two Myanmar men accused of killing a pair of British tourists on a Thai island does not match that found on the suspected murder weapon, a forensics expert witness says.
The testimony casts fresh doubt on the controversial trial of migrant workers Zaw Lin and Win Zaw Tun who are accused of the murder of 24-year-old David Miller and the rape and murder of Hannah Witheridge, 23, on southern Koh Tao island in September 2014.
Both men have pleaded not guilty and face the death penalty if convicted over a case that has tarnished Thailand's reputation as a tourist paradise and seen the police accused of bungling the investigation.
The defence have pressed to retest crucial forensic evidence from the crime scene with a handful of items re-examined by Thailand's Central Institute of Forensic Science (CIFS) including a garden hoe, the suspected murder weapon.
"The DNA on the weapon did not match with the two suspects," Porntip Rojanasunan, director-general of the CIFS, told AFP after testifying at Koh Samui Provincial Court on Friday (local time).
They found the DNA of two "unidentified people" on the hoe, she added.
The battered bodies of Miller and Witheridge were found on the sleepy diving island of Koh Tao on September 15.
Police say Miller had been struck by a single blow and left to drown in shallow surf while Witheridge had been raped and then beaten to death with a garden hoe.
Prosecutors have previously argued that DNA evidence implicates the two Myanmar migrants but the defence has said an under-pressure police force coerced confessions, later retracted, from the pair.
Nakhon Chomphuchat, the lead lawyer for the migrants, said Friday's testimony showed the two men "were not involved with the case as police have accused".
The trial is still under way and a verdict is expected in October.