A man has been arrested in Birmingham by police investigating the Westminster attack, in which five people were killed, including the attacker.
The Met Police said the 30-year-old was being held on suspicion of preparation of terrorist acts.
Another man, 58, arrested in Birmingham on Thursday, remains in custody.
Khalid Masood, 52, killed three people when he drove a car into pedestrians on Westminster Bridge on Wednesday. He then fatally stabbed a police officer.
Police officers then shot dead Masood.
The latest arrest is the 12th to be made by Met Police. Nine people have so far been released without charge.
On Saturday the Metropolitan Police said they believed Masood acted alone.
But they added they were also “determined” to find out whether he had been inspired by terrorist propaganda.
Scotland Yard said it was possible “we will never understand why he did this”.