CCTV images released in Thai murder hunt
CCTV images of what police in Thailand have described as an Asian-looking man have been released in the hunt for suspects in the murder of two Britons.
Investigators said they were looking for a number of people - but have ruled out other British tourists.
David Miller, 24, and Hannah Witheridge, 23, were found dead on a beach in Koh Tao on Monday, having suffered serious injuries in an attack.
The families of both victims paid tribute, saying they would be missed.
In a statement, Mr Miller's family said: "David was an artist by temperament. 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.
"David was very giving to his family and friends and we all adored him. He will be sorely, sorely missed."
Ms Witheridge's family described her as "a beautiful, intelligent, loving young woman who poured joy into the lives of all who knew her".
"She was selfless and caring and made each and every day that little bit more wonderful," the statement added.
"Our family are utterly devastated and shocked by what has happened to our beautiful Hannah. To lose her in the way that we have is beyond comprehension. We are heartbroken and no words can possibly describe how we feel."
Thai police said they had CCTV of Mr Miller and Ms Witheridge walking together, and later footage showing a man - described as being of Asian appearance - who is now the prime suspect.
They said they were now focusing their investigation on the Burmese migrant worker community on the island Koh Tao.
Mr Miller, from Jersey, and Ms Witheridge, from Hemsby in Norfolk, were found close to the beachside bungalows where they had been staying.
A garden hoe believed to have been used in the attack was found nearby.
The bodies have been taken to the capital, Bangkok, for forensic analysis and are expected to be returned to their families after that.
Meanwhile, BBC south-east Asia correspondent Jonathan Head said the police's focus on Burmese migrants would be "quite controversial" as Burmese people were "often scapegoated" for crimes in Thailand.
He said police reinforcements had arrived on the island to help with the investigation, along with a consular official from the British embassy in Thailand.
Police still want to speak to a British man who was with the victims on Sunday evening, but the man is probably not a "major suspect", our correspondent added.
Major General Kiattipong Khawsamang said police were sweeping hotels, bars, businesses and residences of migrant workers on the beach.
At the scene
By Jonathan Head, BBC south-east Asia correspondent
The shocking death of the two young tourists has cast a pall over this scenic island resort.
Locals say they can remember nothing like it happening before.
The police say they are focusing their investigation on Burmese migrant workers on the island.
One senior officer told the BBC he did not believe a Thai person could have committed such a crime.
Police say they have examined CCTV footage showing the two victims leaving a bar late on Sunday night, walking towards the beach - but they were killed in a hidden spot behind some rocks, and no eyewitnesses have yet been found.
Mr Miller was due to return to Leeds University to complete the final year of his masters in civil and structural engineering.
He had gone to Australia in July for a six-week work placement and travelled to Thailand from there for a break.
He was due to leave Thailand on Tuesday.
A statement from Leeds University said support would be offered to help students through "this difficult time".
"He was an excellent student and clearly had a promising career ahead of him," the university said.
Ms Witheridge was about to start the second year of her masters in speech and language therapy at the University of Essex.
In a statement, her personal tutor Teresa Eade and course leader Dr Wayne Wilson described her as a "very strong student".
"She was quiet, reflective, hardworking and resourceful," the statement said.
"She was destined to make an impact in her chosen career in speech and language therapy. She was also someone who had lots of interests outside her studies - she loved to travel and enjoyed horse riding."
The statement added that Ms Witheridge would be "deeply missed by everyone who knew her".
•Small island covering eight sq miles (21 sq km) north of Koh Samui and Koh Phangan in Gulf of Thailand
•Population of about 2,100 and visited by between 400,000 and 500,000 travellers per year
•Popular with backpackers and divers
•Quieter and less developed than neighbouring islands which are known for nightlife and beach parties
Thailand attracts more than 850,000 British and Irish tourists each year, and the UK Foreign Office says 11 British nationals have been murdered in the country since January 2009.
It warns that Western tourists have been victims of "vicious, unprovoked attacks by gangs" on the nearby island of Koh Phangan, but there are no specific warnings for Koh Tao, which is a popular tourist destination.
The Tourism Authority of Thailand said Koh Tao and other beach destinations in Thailand remained safe for tourists.
The country's 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."