A 30-year old man is the 12th person to be arrested by the Metropolitan Police in connection with the Westminster terror attack in London on Wednesday (22 March). The man is being held on suspicion of preparation of terrorist acts, the BBC reports.

Khalid Masood, 52, killed three people - Aysha Frade, Kurt Cochran and Leslie Rhodes - when he drove a car into pedestrians on Westminster Bridge on Wednesday. He then exited the car and fatally stabbed police officer PC Keith Palmer.

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The attacker was shot by another armed police officer and died from his injuries.

Another man, 58, who was also arrested in Birmingham on Thursday (23 March), remains in custody, the broadcaster reports. A 32-year old who was arrested in Manchester is on police bail. Nine people have so far been released without charge.

Police believe that Masood was acting alone and that there was no evidence to suggest that any more attacks were planned.

Deputy Assistant Commissioner Neil Basu said in a statement: "We still believe that Masood acted alone on the day. We must all accept there is a possibility we will never understand why he did this. That understanding may have died with him."

Basu has appealed to the public to come forward with any information that may shed light on Masood's motive. They are trying to find out if he was encouraged or directed by others.

The Islamic State terror group has claimed that one of its "soldiers" carried out the attack.