Three teenagers are being questioned about the murder of Monaghan student Jason McGovern.
Police said a 19-year-old was detained in Omagh, where two other youths, aged 17 and 18, were also arrested by detectives investigating the death.
A man aged 21, also from the Omagh area, was earlier released on police bail pending further enquiries.
The PSNI launched a murder inquiry following Mr McGovern’s death.
They said the 19-year-old died from head injuries sustained when he was attacked twice in Omagh early on Monday morning.
Detectives said Mr McGovern was punched in the head outside a pub and then assaulted again as he was waiting for a taxi back to Emyvale, Co Monaghan.
He was found dead at a friend's house in Emyvale on New Year's Eve.