A group calling themselves members of the IRA have claimed responsibility for sending letter bombs to three London transportation hubs and a Scottish university last week, police said Tuesday.

Police Scotland and the Metropolitan Police said in a joint statement that a media outlet in Northern Ireland received a claim of responsibility on Monday using a recognized code word.

“The claim was allegedly made on behalf of the ‘IRA’,” police said, referring to the Irish Republican Army, which has been in a cease-fire for years.

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“Those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered,” police said in a statement.

Police said they were already investigating possible dissident links to the packages.

The packages bore similarities to devices sent in the past linked to dissident groups associated with Northern Ireland-related terrorism, according to the Guardian.

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All homemade devices – sent to the Waterloo rail station in central London, offices at Heathrow and London City Airports, and the University of Glasgow – had stamps from the Irish Republic and had Dublin as the return address. They also bore stamps issued by Ireland’s post office to mark Valentine’s Day.

Airport operations were not impacted.

One of the devices partially burned when opened and caused a small fire. No injuries were reported

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According to the statement, counter-terrorism police are keeping an "open mind" but stopped short of blaming dissidents for the use of the devices.

Police said they have given extensive devices to businesses and the armed forces to protect against the further use of letter bombs.

The Associated Press contributed to this report.