The IRA has claimed responsibility for parcel bombs sent to major transport hubs in Britain last week.
It also said one parcel, sent to a British army recruitment officer, may not have been discovered yet.
The group claims it posted five devices to addresses in Britain. However, only four have been found.
The claim was received on Monday by a Belfast-based newspaper, using a recognised codeword.
The packages that arrived at Waterloo railway station and offices at Heathrow and London City airports on March 5 and 6 were sent with An Post stamps and had Dublin as the return address, prompting gardai to join the investigation.
The stamps appeared to be those issued by An Post for Valentine's Day last year, featuring a heart motif and the words "Love Eire N".
The senders' addresses were given as Dublin, with two adding Bus Eireann.
Police Scotland said a controlled explosion was carried out as a precaution on a suspicious package found in the mailroom at Glasgow University, after several buildings had been evacuated.
"[The group] claimed that three were sent to 'commercial targets' while the remaining two were posted to British army recruitment officers," according to the newspaper that received the message.
"The group said a device discovered at Glasgow University was intended for a British army recruitment officer who works there."
All were A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened.
No one was injured in any of the incidents and no arrests have yet been made.
A spokesman from the Metropolitan Police said the force is aware of the claim. However, investigations are ongoing.
"We are also aware that those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered," he said.
"Extensive advice has already been issued to relevant businesses and sectors to be vigilant and report suspicious packages to police.
"This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim."
Justice Minister Charlie Flanagan said he has been in touch with UK Home Secretary Sajid Javid about the developments.
"I was deeply concerned to learn of the further developments in the investigation into the incendiary devices which were posted to addresses in the United Kingdom last week," Mr Flanagan said.
"They are the actions of those who are without any vision for a peaceful future in Northern Ireland.
"I reaffirmed to the home secretary our deep commitment to working closely with our colleagues in Northern Ireland and the United Kingdom."