Police are looking into a claim that a group calling itself the "IRA" was behind a series of parcel bombs sent to several UK locations last week.
A news outlet in Northern Ireland received a claim of responsibility using a recognised codeword, the Metropolitan Police said.
Four parcel bombs were discovered last week across three transport hubs in London and one at Glasgow University.
But, according to the claim of responsibility, five parcels were sent, meaning one parcel may not yet have been recovered.
A joint statement from the Metropolitan Police and Police in Scotland said the devices resembled those used in the past by dissident groups in Northern Ireland, meaning they had therefore been "already looking at this line of enquiry".
"However, we continue to keep an open mind and enquiries continue," the statement added.
The parcels, which were sent to Heathrow and City airports, Waterloo station and Glasgow University, all bore stamps from the Republic of Ireland and had return addresses to Dublin.
They also had stamps that were used by the Irish postal service to mark Valentines Day.
"Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police," the joint police statement continued.
"This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim."