British military recruitment centres have increased their security measures after seven failed letter bombings in England were blamed on the IRA.
No group has claimed responsibility for this week's posting of seven envelopes containing gun powder, fuses and other potential bomb parts to recruitment offices in several towns outside London.
British officials blame IRA militants, who are already linked to failed October letter-bomb attacks in Northern Ireland.
Two of this week's threats were mailed from the Republic of Ireland, the rest from within the UK.
Police say they were too crudely designed to have killed anyone.
Most IRA members renounced violence in 2005, but breakaway factions still seek to undermine Northern Ireland's peace settlement.
They have not detonated any bombs in England since 2001.
