According to the release, two other men remain in custody.
Earlier on Friday, police announced that six people detained after the Wednesday terror act in London had been released.
On Wednesday, an attacker, later identified by police as Khalid Masood, 52, hit several people while driving on Westminster Bridge. He then tried to enter the Houses of Parliament, armed with a knife. After stabbing a police officer, who was later confirmed dead at the scene, the attacker was shot.
Several people were arrested by London police on Thursday in connection with the attack.
The identified perpetrator, Khalid Masood, was born in Kent and lived in West Midlands, according to Metropolitan Police. He had several previous convictions for assaults and possession of offensive weapons.
More about: #London #Westminster