All those arrested in connection with the Westminster attack have now been released by police.
Four people were killed when 52-year-old Khalid Masood went on an 82-second rampage in Westminster on 22 March.
Masood ploughed a Hyundai i40 across Westminster Bridge and stormed the Parliamentary estate armed with two knives, fatally stabbing Pc Keith Palmer before being shot dead by police.
The 12 people arrested in connection with the investigation into the attack by Masood will face no further action, police have said.
This includes a 30-year-old man from Birmingham who was arrested on suspicion of preparation of terrorist acts and detained under the Terrorism Act.
Police previously said they now believe the knifeman acted alone but that understanding why he did it may have died with him.
It has since emerged Masood used messaging service WhatsApp seconds before launching the attack - but because messages are encrypted, agencies are unable to see what was said.
Home Secretary Amber Rudd has urged tech companies to stop providing "a secret place for terrorists to communicate with each other".