Osborne was arrested on suspicion of the commission, preparation or instigation of terrorism including murder and attempted murder.
One man died but since he was receiving first aid at the site before the vehicle hit, police say it is not clear whether his death was directly linked. The driver of the van, identified only as a 48-year-old white man, was arrested by the police.
London's Metropolitan Police name one attacker as Khuram Shazad Butt, aged 27 and the second as Rachid Redouane, aged 30.
British police conducted more raids and detained more people in connection with the attack.
The man responsible for killing four people and injuring dozens used techniques that police say were "low sophistication, low tech, low cost."
UK authorities identify the attacker as 52-year-old Briton, Khalid Masood.
British Prime Minister Theresa May gives a sweeping speech in parliament saying "democracy will always prevail."
UK authorities also lowered the death toll from Wednesday's attack to three, excluding the attacker.
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