The man arrested on suspicion of carrying out the Finsbury Park terror attack is 47-year-old Darren Osborne from Cardiff, the BBC understands.
He was held after a van hit Muslims who had been attending evening prayers at a north London mosque.
They had been helping a man who had collapsed. He later died but it is not clear if it was because of the attack.
Mr Osborne was arrested on suspicion of attempted murder and later further arrested over alleged terror offences.
Police are carrying out searches at an address in the Cardiff area.