An Egyptian teenager arrested after a violent rampage in which an Asian man was stabbed to death had entered the Republic through Northern Ireland, Garda believe.
The Garda is trawling through two mobile phones recovered from the teenager as they probe whether the attacks are linked to Islamic terrorism.
The killing of the Japanese man in Dundalk has sparked an international investigation, as Garda national units liaise with colleagues in other countries to determine if the arrested man has links to international terror organisations.
The 18-year-old was in custody last night following the early-morning attacks which left the 24-year-old dead and two Irish men injured.
Two mobile devices seized from the suspect are being examined to establish if any terror-related activity or material is contained on them.
A senior source last night said: "We need to know this man's background. The mobile phones are key to the Garda investigation."
The attacks began shortly before 9am when the Japanese victim was discovered unresponsive at Avenue Road. He was pronounced dead at the scene.
At 9.32am Cian Murphy, who is in his early 20s, was attacked at Coes Road and stabbed in the back. He managed to make his way to his nearby workplace, where colleagues called emergency services.
At 9.40am a third victim was assaulted at Seatown Place, and was struck in the head with a fencing pole.
The injured man went for help at the nearby Smyth's Life Pharmacy. Shop owner Anne McEntegart said: "The wound was pumping, there was a lot of blood.
"It was totally random. He came in here afterwards asking for help.
"We treated the man. I had three to four staff on duty and the blood was pumping from his head."
Detectives believe that the attacker travelled on foot, and he was apprehended a short time later at a derelict property by unarmed officers.
The man was said to be in a very distressed state after his arrest, but was deemed fit to be interviewed after a number of hours. Senior investigators have confirmed that a possible terror link is one line of inquiry the Garda is probing.
Chief Superintendent Christy Mangan said: "A terror attack is a line of inquiry, it's certainly a line that we would look at.
"We will endeavour to establish the suspect's background, who they are, where they've come from and why are they here.
"We're very interested to know why was he in Dundalk, why would he come here, what's the reason, what's the connection."
Detectives believe the arrested man entered Dundalk by crossing the border.
He was stopped and interacted with local officers on January 1, but did not have any identification on him. As a result, he presented at an asylum seekers' centre in Dublin city centre.
However, the man returned to Dundalk on Tuesday and stayed at a derelict property in the town that night ahead of yesterday morning's attacks.