Defence Secretary Michael Fallon has said the working assumption is that the Westminster attack is linked to "Islamic terrorism".
Speaking on Wednesday morning Sir Michael said: "The police are investigating this man, his associates, where he came from, checking urgently whether other people were involved in this.
"That is their assumption at the moment, but they don't have a full enough picture of this man and his known associates, and who may, or may not, have helped him prepare this attack. That work is still going on."
Sir Michael continued that police and investigators "have been working right through the night, looking into his background, how he got hold of the vehicle, where the vehicle has been in the last day or two, and who may, or may not, have helped him."
Asked if he was known to police, Sir Michael said: "I can't confirm that, you will have to ask the police that."
He continued that the security of the Houses of Parliament is to be reviewed, including looking at whether police on the front gates should be armed or not.
The 64-year-old also paid tribute to Pc Keith Palmer who was killed in the attack, saying the police officer has "made the ultimate sacrifice" to keep others safe, and had stopped the attacker getting into Parliament.
All those arrested in connection with the Westminster attack have now been released by police.
Tony Davis was the first civilian to reach Pc Palmer after he was stabbed during the Westminster attack.
Defiant Londoners put on a show of solidarity for the victims of the Westminster terror attack at a vigil in their honour.