BANGKOK – Thai police investigating the murder of two British tourists said on Sunday that they were no closer to an arrest as frustration grows over the lack of a breakthrough.

The bodies of Hannah Witheridge, 23, and David Miller, 24, were discovered on Sept. 15 on Koh Tao island, popular with divers who come for its coral reefs and beach parties.

Police spokesman Lieutenant Colonel Kissana Phatanacharoen told Reuters that "hundreds of DNA samples" had been collected.

"The Royal Thai Police need time to complete their investigation particularly given the intense international interest," Kissana said. "Police are no closer to an arrest but the investigation is close to being completed.

"We are narrowing down our list of suspects. Hundreds of DNA samples have been taken on Koh Tao," he added.

Police have offered a 700,000 baht ($21,650) reward for any information leading to an arrest.

Post-mortem examinations by Thailand's forensics department showed that Miller died from drowning and blows to the head while Witheridge died from severe head wounds.

Police said on Friday they had taken DNA samples from a group of Thais who had taken part in a football tournament in the area.

DNA samples were also taken last week from a group of workers from neighbouring Myanmar. They have since been eliminated as suspects.

Authorities are under intense pressure to solve the case which has garnered huge interest both domestically and abroad.

Some have criticised the investigation saying that the crime scene was not sealed off quickly enough and that potential suspects were allowed to leave the island.

Kissana defended the investigation.

"We realise the public wants answers. We have poured many resources into the investigations and what we are asking for now is the time and space to investigate thoroughly so that the culprits can be arrested quickly."