A man has been arrested on suspicion of posting an offensive tweet about the Finsbury Park attack in London.
The son of one of the owners of the Rhonda Cynon Taff company whose van was used in Monday’s attack is in custody.
Richard Evans, 37, allegedly tweeted: “It’s a shame they don’t hire out steam rollers or tanks could have done a tidy job then.”
His father, Lee Evans condemned the remarks “in the strongest possible terms”.
South Wales Police said a 37-year-old man was being held on suspicion of displaying threatening, abusive, insulting written material with intent that is likely to stir up racial hatred.
Lee Evans said: “I want to condemn in the strongest possible terms these ill-considered Twitter comments, which in no way reflect my own view or indeed those of anyone else in the family.
“The attack in Finsbury Park was shocking and cowardly. Together with all the staff at Pontyclun Van Hire, I am doing everything I can to assist the Metropolitan Police in their inquiries.”
Darren Osborne, 47, from Cardiff, is being held on suspicion of attempted murder and alleged terror offences.
The attack happened as an ill man was receiving first aid from the public near the mosque, where people had been saying Ramadan night prayers.
The van was apparently driven at people who were helping him.
The man died and nine people were taken to three London hospitals, while two others were treated for minor injuries.
Witnesses said they heard the driver, who was detained by members of the public before police arrived, shout: “I am going to kill Muslims.”
One of Mr Osborne’s neighbours said they thought they had seen him sleeping in a van on Saturday night.
Another said he had been living in a tent in woodland after being kicked out of the family home.