The Department of Special Investigation (DSI) will seek DNA samples from a man in Japan for comparison with genetic material found on the clothing of a Japanese woman slain in Sukhothai Historical Park in 2007.
Justice Minister Somsak Thepsutin, DSI chief Pol Lt Col Korrawat Panprapakorn, and Worawee Waiyawut, director of the Genetic Material Division at the Central Institute of Forensic Science (CIFS), on Thursday announced the move at a joint media conference on the unsolved murder of Tomoko Kawashita. Japanese embassy officials also attended.
Mr Somsak said the DSI took over the murder case in 2013, but despite their best efforts the investigation ground to a halt six years ago due to a lack of evidence pointing to the killer.
The DSI later reopened the case following information that a man who died in 2010 was a suspect in the unsolved murder of the Japanese tourist.
Investigators tried to extract genetic samples from the cremated remains of the man for comparison with genetic traces found on the victim's trousers. They were unsuccessful, the minister said.
"The investigators later learned that the suspect had a son. They conducted a DNA test on his son for the Y chromosome to establish a father-child link. They also found out the dead man had a half-brother and a nephew working in Taiwan.
"Investigators and forensic experts went to Taiwan to collect DNA samples and Y chromosomes to prove their relationship with the dead man.
"However, they found the genetic material of the three men did not match with the Y chromosome found on the trousers of the victim,'' the minister, who is from Sukhothai province, said.
Dr Worawee said investigators had sent the victim's white short-sleeved T-shirt, long-sleeved brown blouse and blue trousers to the Central Forensic Science Institute for another round of tests.
New procedures using new chemicals found genetic material in 22 places, more than the previous test, which found only 16. This could prove useful in shedding light on the killer's identity, he said.
Forensic experts shared the view the genetic traces pointed to a Japanese or Korean person, he said.
Pol Lt Col Korrawat said DSI investigators were working closely with the Japanese embassy. It was hoped Japanese authorities would get court approval to ask another man in Japan to give a DNA sample for comparison with the genetic material extracted from the victim's clothing.
"On top of this, the investigators will further investigate whether other Japanese or Korean nationals visited Sukhothai Historical Park on the day the crime occurred,'' he said.