BANGKOK (AP) — An expert witness at the trial of two Burmese workers accused of killing two British tourists in Thailand testified Friday 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 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.

Porntip testified that the hoe contained DNA from two males, but not the suspects.

The point of her testimony seemed to be to discredit the police investigation and cast doubt on the rest of the prosecution case. She is a celebrity in Thailand who has written books about her work. However, she is also controversial because her findings in in several high-profile cases contradicted those of the police, leading to a long-running feud.

The two defendants, Win Zaw Htun and Zaw Lin, both 22, were arrested in early October, after police carried out DNA tests on more than 200 people on Koh Tao, a scenic island in the Gulf of Thailand known for its scuba diving. The trial is taking place on the bigger nearby island of Koh Samui.

Human rights groups repeatedly called for an independent investigation and raised concerns that the men might be scapegoats, partly because as migrant workers they are discriminated against.