DNA tests have confirmed the blood of the Silpakorn University student killed with a screwdriver was found on the pants of the son of a deputy chief investigator of Kalasin police.
It was also revealed the Crime Suppression Division had still not called another suspect in the case, a politician's nephew, in for questioning.
DNA checks were made of blood stains found on clothing taken from Dechathorn Moonmanee's room in Bangkok, which police searched on Wednesday. Mr Dechathorn, also known as Bright, is a son of Pol Lt Col Sakdinant Moonmanee, deputy chief investigator of Kalasin province.
The DNA comparisons confirmed that blood found on the suspect's pants was from murdered student Teerapong Thitathan. It would be important evidence in the case, Pol Gen Srivara Ransibrahmanakul, deputy police chief, said on Thursday.
Investigators also questioned Teerapong's mother, Areerat Chomloke, Pol Gen Srivara said. Mrs Areerat raised the family's concern that arrest warrants had not been issued for three more suspects in the case, one of them a young nephew of a local politician, Pol Gen Srivara said.
The reference was to a politician in Phetchaburi province, where the murder occurred at a university dormitory late last month.
Pol Gen Srivara did not confirm if the suspect in question was the relative of a politician.
He said that based on answers from questioning of the detained suspects, one of the three additional suspects had not been present. Police would investigate whether that person helped mastermind the attack.
Police would need to further question witnesses to establish links between him and the case, as he was not at the crime scene.
Investigators had learned that each of the suspects had been in a fight with friends of the slain victim, Pol Gen Srivara said. He did not confirm if the suspect in question was the relative of a politician.
The Kalasin police officer's 21-year-old son was among 16 suspects alleged to have attacked Teerapong and his friends at their dormitory in Cha-am district of Phetchaburi about 1.30am on Feb 25.
The murdered student was fatally stabbed in the right side of his head with a screwdriver. The victim suffered several other injuries, but a post mortem examination confirmed the screwdriver killed him.
Two other students living in the dormitory were also severely hurt during the attack.
The police officer's son, who has reportedly confessed, was refused bail and remanded in custody. The others were released on bail of 700,000 baht each.
A source said the motive for the attack was a quarrel between Mr Detchathorn's group and the victim's friends over Mr Detchathorn's girlfriend, who is among the suspects.
Mrs Areerat received 110,000 baht compensation from the Justice Ministry on Thursday.