Police are investigating whether a convicted terrorist managed to access any sensitive areas of the London Underground (LU) while working there.
Kamel Mostafa, son of jailed Islamic cleric Abu Hamza, was jailed in Yemen in 1999 for planning a bomb campaign.
He worked for a sub-contractor of Tube maintenance company, Tube Lines.
British Transport Police (BTP) said it was working with LU to "reassure both staff and passengers" and check any future changes to procedures.
It is understood Mostafa's previous convictions were not brought to light during his recruitment and he has since been dismissed.
A BTP statement said: "We have worked closely with LU to identify the areas where Mohammed Mostafa may have accessed, to reassure both staff and passengers BTP has undertaken operational activity in these areas."
A spokesman for Transport for London (TfL) said on Wednesday: "We will review this case to ensure that the relevant checks were undertaken, and ensure any further procedures which may be possible and necessary."
Abu Hamza was jailed for seven years in February for incitement to murder and inciting racial hatred. He is currently appealing against his conviction.
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