Police in Thailand have admitted the investigation into the brutal murders of two British tourists is proving "quite difficult" as they revealed they do not know how many people were involved in the killings.

Five days after the bodies of Hannah Witheridge, 23, and David Miller, 24, were found with severe head wounds on a beach on the island of Koh Tao, a spokesman for Royal Thai Police insisted officers were "working around the clock to find the culprits" amid fears the investigation has stalled.

Post-mortem examinations revealed Ms Witheridge, from Great Yarmouth, Norfolk, died from head wounds while Mr Miller, from Jersey, died from severe blows to the head and drowning. A bloodstained garden hoe, believed to be the murder weapon, was found nearby.

Police Colonel Kissana admitted it was "difficult to say" if the killers were still on Koh Tao and police were unable to confirm if there was more than one murder weapon.

He said: "We can't rule out any possibility. We have not come up with an exact number of suspects. Last night we conducted a crime scene reconstruction to get a sense of what happened."

He said 150 officers had been deployed on the island and senior police officials had met Miss Witheridge's "deeply distressed" family.

Meanwhile, two British brothers who were questioned by police have been told they are free to return home, police have confirmed.

Christopher and James Ware, friends of Mr Miller who had been speaking with officers but were never detained or declared to be suspects, were told they could fly back to Jersey.

Police say they are still looking for three Westerners who were seen playing a guitar near the scene on Koh Tao's main Sairee beach.