An expert witness at the trial of two Burmese workers accused of killing two British tourists in Thailand has testified that DNA found on what police say was the murder weapon does not belong to the defendants.

Autopsies showed that 24-year-old David Miller and 23-year-old Hannah Witheridge suffered severe head wounds and that Witheridge was raped in September last year. The prosecution says DNA found in semen links the two accused migrant workers to the crime.

The testimony Friday by head of Thailand's Central Institute for Forensic Science Porntip Rojanasunand seconded criticism that police mishandled evidence, including the hoe the authorities say was the murder weapon.

The defendants say they were tortured into making false confessions.