Scotland Yard sources emphasised that following the release of the people - believed to be three men and two women arrested during raids in London and Hertfordshire last Tuesday - investigations were 'continuing and ongoing'.
Those arrested were questioned at Paddington Green high-security police station, and released just hours before the seven-day deadline on detentions under the Prevention of Terrorism Act.
Forensic scientists are still examining the large quantities of bomb-making equipment - including several tonnes of materials for making high explosives - three vans, and other vehicles which were seized at the same time. The work is expected to take several weeks.
Anti-Terrorist Branch sources say the IRA was planning to explode the devices inside the vans at strategic points around London in a similar fashion to those which were detonated outside the Baltic Exchange in the City of London and at Staples Corner in north London in April. The raids followed a lengthy surveillance operation, and a tip-off from a member of the public who was suspicious about the sale of a van.
Forensic scientists are examining the remains of an incendiary device which burst into flames in a branch of W H Smith in Bournemouth, Dorset.