The death of a teenager on New Year's Eve is being treated as murder, police have said.
Jason McGovern, 19, from Tydavnet, Co Monaghan, died after being assaulted during a night out in Omagh, Co Tyrone, on Sunday.
The detective leading the murder hunt said post-mortem examination results revealed he died from a head injury.
Mr McGovern, who cared for his mother who suffers from multiple sclerosis, was injured after being punched at a bar on John Street in Omagh and then hit again two hours' later outside a nightclub on the Kevlin Road.
It is understood Mr McGovern may have been trying to break up a fight.
He managed to make his way home to the border village of Emyvale, Co Monaghan, where he was believed to have been staying with a friend.
His body was found just after midday on New Year's Eve.
The murder investigation is being led by the PSNI and supported by Gardai.
Detective Chief Inspector Richard Harkness appealed for taxi drivers and motorists at the Weigh Inn car park, Omagh, to come forward.
The detective is especially keen to speak to two middle-aged men who helped Jason into his taxi with his friend after the assault.
The men are not linked to the assault.
Police are also trawling through CCTV footage from the Weigh Inn car park and Omagh town centre.
The PSNI are keen to speak to a man of stocky build with ginger or blond-coloured hair as part of their inquiries.
Police believe Mr McGovern's death was the result of a single punch. To date such incidents have claimed the lives of 20 people in Northern Ireland since 2004.
Mr Harkness said: "This is an assault that had no apparent motive other than young men out on a night out and fuelled by alcohol."
It is understood that Mr McGovern had travelled to Omagh by taxi with a group of friends but the group became separated during the evening.
Shortly before 2am, Mr McGovern and a friend were outside the Terrace Bar where there was an altercation with a gang of five men. Punches were thrown and Mr McGovern was struck several times in the head and upper body.
He and his friend then moved away, along Kevlin Road towards a taxi which was waiting outside the Weigh Inn. They were followed by at least three of the five men and, in the Weigh Inn car park, Mr McGovern was struck once more in the face. He collapsed to the ground and had to be helped into the taxi by two other men who were not involved in the row.
Mr Harkness said: "I have a specific appeal, and that is to any pedestrians or motorists who were in the area of the Terrace Bar of Sally O'Brien's nightclub at approximately 1.55am and witnessed the assault and the people involved in it, or the two males who were at Kevlin Road and assisted Mr McGovern to his feet and helped get him into the taxi. I need those people to come forward and speak to me.
"There is some CCTV in that area; however, I need to establish what took place and what was said between the assailants and the victim."
The detective also made a direct appeal to those involved in the attack.
He said: "Don't delay, come forward. There are two sides to every story. I want to establish the facts.
"If you were involved, come forward to detectives in Omagh and we will interview you to establish the facts."
Police are particularly keen to speak to a man who is in his mid-20s, about 5ft 4in and of stocky build. He was wearing a white T-shirt and was involved in the incident outside the Weigh Inn.