Police are currently holding five people in custody over the London tube bombing, and have described the investigation as "fast-moving." ? AFP

LONDON: British police on Thursday released a man arrested in connection with the bombing of a London Underground train last week, leaving five suspects in custody over the attack.

The 21-year-old was released with no further action following his arrest late Saturday in Hounslow, west London, the Metropolitan Police said in a statement.

Dean Haydon, the force's head of counter-terrorism, said the probe into the bomb attack at Parson's Green station "continues to be a fast-moving investigation" and searches are continuing at four addresses.

Placed on a packed train during morning rush hour, the improvised device injured 30 people when it went off despite apparently malfunctioning.

The attack in Parson's Green, a quiet residential area, caused panic among commuters and some were hurt in the stampede as people fled the scene.

Police made a first arrest a day later when they picked up an 18-year-old man at Dover, a port in the southeast which serves as a gateway to Europe.

The investigation subsequently shifted to Newport in Wales, where three men aged between 25 and 48 were arrested on Tuesday and Wednesday.

Then a 17-year-old suspect was arrested in the early hours of Thursday in Thornton Heath in south London.

The tube bombing was the fifth terror attack in Britain in sixth months -- four of them in London and one in Manchester -- with the bloodshed claiming 35 lives.