Police are facing questions over why they did not act sooner against radical Muslim preacher Abu Hamza al-Masri.
Detectives were "very alert" to the activities of the cleric - jailed on Tuesday for seven years for inciting murder and racial hatred - in 1999. But the 47-year-old, from London, was not arrested until 2004.
Anti-terror police say evidence was sent to prosecutors "on several occasions" but no action was taken by the Crown Prosecution Service.
UK police had interviewed Abu Hamza during 1999 over alleged involvement in terror plots in Yemen, but no charges were brought.
He was eventually arrested in 2004 following an extradition request from the US, but charged five months later with offences relating to his activities in the UK.
On Tuesday, Abu Hamza was convicted of inciting murder and racial hatred and possession of a terrorist document, after a trial at London's Old Bailey.
The preacher could still face extradition to the US on terrorism charges when he is released from jail in Britain.
Labour MP Louise Ellman said: "I think it was very wrong his activities were ignored for so long, but I am glad at long last he has been brought to justice."
Abu Hamza has been blamed for radicalising Muslims who prayed at Finsbury Park Mosque, in north London, where he was imam until 2003.
A police search of the mosque that year led to the discovery of forged passports, CS gas, knives and guns and it was closed down.
Egyptian-born Abu Hamza continued to preach outside the mosque, but following his arrest in 2004 more than 3,000 audio cassettes and 600 videos were found of speeches intended for wider distribution.
And a terror manual - an encyclopaedia of Afghani Jihad - found at his west London home listed Big Ben, the Eiffel Tower and the Statue of Liberty as possible targets for an attack, reports the AP.