Story highlights Police say DNA test don't match evidence from female victim's body

DNA was taken from two British brothers and 11 other men

Autopsy: Slain woman had signs of sexual intercourse, getting hit in head

The male victim had water in his lungs, was hit by hard object, police say

Thai police are still searching for suspects in the killings of two British tourists on an idyllic resort island after DNA tests on more than a dozen men found no matches with evidence from the female victim's body.

Provincial police chief Kiatthipong Khaosumarng said that experts carried out the tests on DNA samples from 11 migrants working on the island and two British brothers, one of whom had shared a room with the male victim.

No one has been arrested yet over the killings on the island of Koh Tao, nor has anyone been officially named a suspect. Police had previously asked the two British brothers, Christopher and James Ware, not to leave Thailand, saying they were "under watch."

Kiatthipong declined to comment on reports that the two brothers have now been allowed to leave Thailand.

Police are now trying to find other people who were caught on surveillance video the night the two tourists -- Hannah Witheridge, 23 and David Miller, 24 -- were killed, he said.

An autopsy revealed both victims had been hit by a hard object, according to Maj. Gen. Pornchai Suteerakhun, the head of the Police Hospital's Institute of Forensic Medicine.

Miller was hit and had scratches on his back, in addition to having water in his lungs consistent with drowning, an autopsy showed. Witheridge was hit several times on her head and face and her body had evidence of sexual intercourse, according to her autopsy.

Pornchai didn't confirm or deny that authorities believe Witheridge was raped.

Kiatthipong said that two sets of DNA have been found on Witheridge's body, indicating at least two culprits in her death. Neither set matches that of Miller, he said.

CCTV image released by police.

Thai officers work near Koh Tao beach.

Family: 'A beautiful, intelligent, loving young woman'

Police previously said they had ruled out robbery as a motive.

Miller and Witheridge were last seen with friends at a bar in the early hours of Monday before leaving together, according to Kiatthipong. Police have released a grainy image from video outside that bar, adding that they wanted to interview a person -- who they described as an Asian man -- seen in it.

The victims were discovered partially undressed with severe injuries to their heads, and a hoe with blood on it was found near the bodies, police said.

The deaths have shaken residents on popular holiday island, which takes up only 21 square kilometers (8 square miles) amid bright blue waters in the Gulf of Thailand.

Authorities said it was the first homicide case on the island -- known for its diving locations, white-sand beaches and vibrant nightlife -- in at least eight years.

The confusion and heartache there, though, pales compared to that felt now half a world away in Briton, where two families are coming to grips with their loved ones' deaths.

In a statement issued by the British Embassy in Bangkok, Witheridge's family said she was "a beautiful, intelligent, loving young woman who poured joy into the lives of all who knew her" and they were "utterly devastated and shocked by what has happened."

Miller's family described David Miller as "an artist by temperament, so talented.

"He had a creative eye that he carried with him through life and in his degree. He was hard-working, bright and conscientious, with everything to look forward to."