A van driven into worshippers near a north London mosque belongs to a Welsh hire firm.
Photographs of the white van involved in the incident in north London, which is being treated as terrorism, feature logos of 'Pontyclun Van Hire'.
Pontyclun Van Hire says it is "shocked and saddened" by the attack and is cooperating fully with the police investigation.
One man has been arrested after the incident outside the Muslim Welfare House near Finsbury Park mosque, which killed one man and injured 10 others.
The Metropolitan Police has confirmed the incident is being treated as a terrorist attack.
The Secretary of State for Wales, Alun Cairns MP, has confirmed that South Wales Police is working in partnership with the Metropolitan Police "as the investigation into the abhorrent attack at Finsbury Park gets underway."
The van driver has been described by eyewitnesses as a "large white man".
He was detained by members of the public after the attack in Seven Sisters Road.