The test results from the semen found in the female victim's body indicated the suspects were Mongoloid and not Caucasian.
The results helped narrow down the search, said Pol Gen Somyot, who will become the national police chief on October 1.
Two Britons, Hannah Witheridge, 23, and David Miller, 24, were found dead on a beach on Koh Tao in Surat Thani province with severe head injuries on Monday morning (September 15).
Earlier, the autopsy on Ms Witheridge found semen from two people but subsequent DNA tests found no matches to any of the seven people detained — six Myanmar labourers and one Briton — shortly after the murders.
A cigarette butt found near the bodies on Friday (September 19) contained the DNA of Ms Witheridge and another person. It was believed the second person was someone she knew because the cigarette was shared.
Pol Gen Somyot was in Koh Tao earlier today (September 20) to follow up on the investigation.
He said that after the reconstruction of the crime scene on Friday, the case had progressed and the police had not ruled out anyone. Officers even took DNA samples of more than 20 local women, British media reported.
People on the island from tourists to fishermen and workers are being investigated, he said.
Police on Friday asked the US Federal Bureau of Investigation (FBI) to help with DNA testing to identify racial markers but they later cancelled the request after succeeding in conducting the tests.
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