The man suspected of driving a van into Muslim worshippers near a north London mosque is Darren Osborne from Cardiff, the BBC understands.
One man died at the scene and 11 people were injured after the van mounted a pavement near Finsbury Park Mosque.
An address in Cardiff is being searched while the van belonged to Rhondda Cynon Taff hire firm Pontyclun Van Hire.
Police said a 47-year-old man was held on suspicion of attempted murder and alleged terror offences.
Father-of-four Mr Osborne's sister Nicola, speaking outside a house in Weston-Super-Mare, where family members gathered said: "I am very sorry for what's happened."