She told MPs he had been investigated some years ago but was not part of the current intelligence picture.
The so-called Islamic State group has said it was behind the attack.
Eight arrests in London and Birmingham followed the attack that has left four dead - PC Keith Palmer, Aysha Frade, Kurt Cochran and the attacker.
Seven of the injured are still in hospital in a critical condition. A further 29 had been treated in hospital, police said.
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2 killed in London 'terror' attack (Pics)