The bodies of David Miller and Hannah Witheridge were found early on Monday (local time) on a beach on Koh Tao, a southern island known for its coral reefs and diving. Both bodies were found naked.
Police found the DNA of two people in semen taken from Witheridge's body, the chief of Thailand's forensic police, Pornchai Suteerakune, told Reuters on Thursday. The DNA does not match Miller's, nor any of the samples taken from those questioned over the murders.
"Tourists think that Thailand is beautiful, safe and that they can do anything they want here. That they can put on their bikinis and go anywhere they want. I ask, can you get away with wearing bikinis in Thailand? Unless you are not beautiful?" Prayuth said in a televised speech on Wednesday.
The two deaths bring the number of UK nationals murdered in Thailand to 13 since 2009, the British Consulate said on Wednesday. Some 800,000 Britons visit Thailand each year, among more than 25 million foreign visitors.
"I am sorry with what I said and if it has caused any ill-feelings," Prayuth told reporters. "I just wanted to warn tourists that we have different traditions and they have to stay on their toes."
The autopsy also revealed that water was found in Miller's lungs suggesting that he could have drowned, Pornchai said.
Both victims were found with head wounds that had led police to conclude initially they were bludgeoned to death.
The victims' bodies have been handed over to family members in Thailand for repatriation.
