The 15-year-old Co Antrim teenager arrested over the TalkTalk hack has been released.
The teen was arrested at around 4pm yesterday by PSNI officers on suspicion of Computer Misuse Act offences.
15 year old male arrested in connection with alleged theft of data from firm Talk Talk has been released on bail pending further enquiries.— PSNI (@PoliceServiceNI) October 27, 2015
He was released after 9am on Tuesday on police bail pending further enquiries.
Telecommunications giant TalkTalk was hit earlier this month by a "significant and sustained cyber attack".
The phone and broadband provider, which has more than four million customers, said banking details and personal information could have been accessed.
A criminal investigation was launched on Thursday.
The arrest is part of a joint investigation by the Metropolitan Police Cyber Crime Unit (MPCCU), the PSNI's Cyber Crime Centre and the National Crime Agency.
TalkTalk is facing a loss of £75m in revenue from the cyber attack, in which hackers allegedly broke into its computers and accessed customers' personal data and bank details.
It has been reported that the hackers demanded a ransom from the company.
It was not known last night whether police are seeking anyone else in relation to the cyber attack, which had originally been claimed by Islamist extremists.
A spokesman from the Metropolitan Police last night said: "There have been no other arrests at this stage and it is difficult to speculate where the investigation will go as enquiries are ongoing.
"At approximately 4.20pm, officers from the PSNI, working with detectives from the MPCCU, executed a search warrant at an address in Co Antrim.
"At the address, a 15-year-old boy was arrested on suspicion of Computer Misuse Act offences.
"A search of the address is ongoing and enquiries continue."
Policing Board member Jonathan Craig told the Belfast Telegraph that he believes this to be the most significant arrest in terms of a hacking investigation to take place in Northern Ireland.
He also said it raises questions for TalkTalk of - if the boy is proven to have taken part in the hack - how a teenager in Co Antrim could have been able to infiltrate a major company.
North Antrim MP Ian Paisley said he had received a briefing from the PSNI about the case.
"I welcome that the PSNI have secured evidence and been able to make this arrest," he added.
"I know that in this instance they are working hand in hand with the Metropolitan Police as they investigate a crime which has caused concern to people right across the United Kingdom. I will be meeting with police again to seek updates on the progress of the investigation."
Meanwhile, TalkTalk moved to allay concerns of existing and former customers over the data breach, saying: "We know this has been a worrying time for customers and we are grateful for the swift response and hard work of the police. We will continue to assist with the ongoing investigation."