British police said they released on Saturday the final person being held over last week's terror attack outside the British parliament, which killed four people.
A 30-year-old man arrested in the central city of Birmingham on March 26 on suspicion of preparation of terrorist acts was the last of the 12 arrested over the attack to be released.
"All those arrested in connection with the terrorist attack in Westminster on Wednesday, 22 March have now been released with no further action," Scotland Yard said a statement.
Four people were killed and dozens more injured when 52-year-old Muslim convert Khalid Masood ploughed through a crowd of pedestrians and fatally stabbed a policeman just inside the gates of the British parliament before being shot dead.
The Islamic State group claimed that one of its "soldiers" carried out the attack, the worst in Britain since the July 2005 bombings which left 52 people dead, but police say there is "no evidence" of a link.