Authorities in Thailand are reporting that they are looking for at least two suspects in the Thai killings.

The bodies of Hannah Witheridge and another British tourist David Miller were found on Koh Tao on Monday. However, only Ms Witheridge's blood was found on a garden hoe at the scene of the beach attack on the island of Koh Tao.

Now, investigators are scouring the area for the blunt metal object used against Mr Miller.

DNA testing on a cigarette butt had earlier been matched to semen found on Ms Witheridge's body, and officers say they have established that more than one person had smoked the cigarette.

Maj Gen Apichal Tiamataya told the Phuket Gazette: "From the available evidence, we believe there were more than two attackers.

"I am well aware that the murders have affected Thailand's reputation and travellers' confidence. I'm asking for more time as I want to work on a step-by-step basis."

The cigarettes found metres away from the Britons' bodies appear to be the focus of the investigation. They have been identified as a little-known brand apparently smoked by migrant workers in the region.

Royal Thai Police has already been forced to release 12 suspects after their DNA samples failed to bring a match - and DNA testing on a strand of blonde hair found in Ms Witheridge's hand also proved inconclusive, according to the Bangkok Post.

Despite the slow start to the investigation, Lt Gen Panya Mamen insisted he was confident that the attackers will be brought to justice.

"There is a lot of evidential information that we cannot reveal to the media right now," he told the Phuket Gazette, while confirming the police search was being expanded to islands surrounding Koh Tao.