Until now, it has been considered possible that the killer or killers could have been of European background.
The DNA tests were made on sperm taken from the body of one of the victims of Monday's brutal slayings on the holiday island in the Gulf of Thailand, Hannah Witheridge, 23.
The refinement in the pursuit of the killer or killers of Ms Witheridge and David Miller, 24, comes as Thai police change their minds about asking for FBI help and Prime Minister Prayuth Chan-ocha writes letters of condolences to his British counterpart, David Cameron, and the victims' parents.
The suggestion of involving the American agency came yesterday as senior Thai police sought better technology to enable them to make the most of the forensic evidence in the murder mystery.
Attention is now focused on a man who appears to be of Asian extraction, caught on security cameras near the crime scene early on Monday morning but not yet identified.
Twelve other people, including two other British tourists, have been cleared of suspicion by DNA comparisons so far.
Here's what Prime Minister Prayuth wrote in his letters:
The Right Honourable David Cameron,
Prime Minister of the United Kingdom of Great and Northern Ireland
Dear Prime Minister,
On behalf of the Government and the people of Thailand, I would like to extend to you, and through you, our deepest condolences on the tragic death of two British nationals, Ms. Hannah Victoria Witheridge and Mr. David William Miller, on Koh Tao on 15 September 2014. Our thoughts and prayers are with the bereaved families of Ms. Witheridge and Mr. Miller as they mourn the tragic loss of their loved ones.
Please rest assured that my Government is fully committed to conducting on an urgent basis, a full investigation into the incident in order that each and every one of those who committed this heinous crime is brought to justice. Relevant Thai authorities have been working closely with the British Embassy in Bangkok in rendering all the necessary assistance to the families of Ms. Witheridge and Mr. Miller.
I wish to take this opportunity to reaffirm that my Government attaches the highest priority to protecting the safety of foreign nationals, both residing in and visiting Thailand. We have immediately further reinforced security measures which are popular tourist destinations countrywide in order to ensure utmost safety for foreign and British citizens alike.
General Prayut Chan-o-cha
Prime Minister of the Kingdom of Thailand
Dear Mr. and Mrs. Miller,
I wish to express my deepest condolences on the tragic death of your beloved son. I know that in this moment of profound sadness, no amount of words can ease the tremendous pain and grief your family much be going through.
We in Thailand are all deeply shocked by this very tragic incident and I want to assure you that the Royal Thai Government will spare no effort in brniging those who committed this heinous crime to justice.
In this moment of deep sorrow and mourning, please be assured that the Royal Thai Government is ready to render its fullest assistance.
Prime Minister of the Kingdom of Thailand.