A 58-year-old man is still being held by police over the Westminster attack, as two others are released without charge.
Police said a man, 27, from Birmingham, who was earlier in custody, and a 39-year-old woman who was on bail have been released with no further action.
Four people died and 50 were injured when Khalid Masood drove his car into pedestrians before stabbing a police officer. He was shot dead by police.
Fourteen people remain in hospital.
PC Keith Palmer was guarding Parliament when he was stabbed by Masood. He was unarmed, but the Metropolitan Police have confirmed that he was wearing a protective vest.
A post-mortem examination heard that there was no obvious evidence the vest had been damaged or penetrated, but it had been cut off when emergency first aid was given, the Met said.
It added that the coroner had requested that the provisional cause of death was not released at this stage.
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Eleven people were initially arrested over the incident.