In an incident in Barcelona, a white van sped down the popular Las Ramblas avenue packed full of tourists on August 17, knocking people down and killing 13.
Then just eight hours later, attackers struck again at a seaside resort of Cambrils in Spain where a car rammed into pedestrians, injuring six civilians and a police officer. One of the civilians later died.
Federal Bureau of Investigation (FBI) documents seen by 'The Sunday Times' reveal that surveillance technology dispatched to Spain is believed to be linked to the development by the terror group of weaponised drones routed through the UK.
US court documents seen by the newspaper show the lengths to which individuals involved with the Wales companies went to mask their alleged activities.
One of the firms — which shipped surveillance equipment to Madrid, Spain — appears to have been set up using the identity of a bogus director and shareholder called "Peter Soren".
The name is believed to be an alias for Siful Sujan, a Bangladesh-origin IT expert and businessman behind the companies who left south Wales three years ago with his family to join the ISIS in Syria.
He was later killed by a US drone strike in Raqqa, the terror group's de facto capital.
Sujan and his associates originally set up the Ibacs network of companies to offer website and printing services to restaurants and takeaways.
They operated out of an office on Alexandra Gate business park at Tremorfa in Cardiff.
But a probe involving the FBI in the US and British anti-terrorist police discovered that some of the companies became involved in more sinister activities after Sujan headed to Syria, where he was a senior figure in ISIS hacking operations and weapons development.
One of the Cardiff-based firms, Ibacstel Electronics, was used to send a total of $7,700 in 2015 to an ISIS supporter in Maryland, US, called Mohamed Elshinaway, 32, who pleaded guilty to terrorism charges last week.
In July 2015 a company called Advance Technology Global (ATG) was created and registered at the Alexandra Gate business address, the FBI claims.
Sujan came to the UK to study in 2003 and was followed by his wife, Akter, two years later.
They had a son, Amanul, in 2011 and lived in Pontypridd, but later moved to a rented townhouse in Cardiff, close to the Alexandra Gate business park.
The couple briefly moved back to Bangladesh before travelling to Turkey with their son.
Ibacstel Electronics was dissolved in March 2016 and ATG was wound up eight months ago.
Other Ibacs-linked firms continue to operate out of a new office in Newport but there is no suggestion that they are involved in any wrongdoing, the newspaper said.
The FBI disclosures will raise fears about the likelihood of further attacks in Europe.