Police in Thailand say they are focusing on migrant workers in the hunt for the person responsible for the murder of a British man and woman on the island of Koh Tao.

The bodies of David Miller, 24, from Jersey, and Hannah Witheridge, 23, from Great Yarmouth, Norfolk, were discovered semi-naked on a beach at the popular divers' destination on Monday. A bloodstained hoe, believed to be the murder weapon, was found nearby.

Police have released still images from CCTV footage in the area, one of which shows what police said were the two victims walking hand-in-hand.

They said a man pictured in a later image was considered a prime suspect, but that they were following a number of leads.

"We are focusing on migrant workers because of the surrounding witnesses and evidence, including the video footage," Major General Kiattipong Khawsamang said. "We are sweeping hotels, bars, businesses and residences of migrant workers on the beach to find the suspect."

Witheridge's father, Tony, said his family was distraught at receiving "the most devastating news". On Monday, there were reports that police were looking for a British man who may have known Miller, in connection with the murder.

It was not thought the victims were a couple, but had met recently on the island. Their bodies were found on a rocky section of the shore around 100 metres from the scene of a beach party.

Police said the pair had sustained horrific injuries – the man was cut on the back and the side of his head, while the woman was cut on her face.

Local people gathered on Monday night for a candlelit vigil in memory of the victims.

The murders could badly damage tourism in the country, which has already been affected by a deep political crisis and the introduction of martial law.

The prime minister, General Prayuth Chan-ocha, said: "This should not have happened in Thailand. It will affect our image in the eyes of international countries."

The defence minister, Prawit Wongsuwan, said police hoped to make an arrest as soon as possible.

A British Foreign Office spokeswoman said on Monday that staff from its Bangkok office were on their way to Koh Tao.

"We are aware of the death of two British nationals on 15 September on the island of Koh Tao in the Gulf of Thailand. The embassy are urgently seeking information from the local authorities and consular staff stand ready to provide assistance to friends and family at this tragic time," she said.