The 24-year-old man, who was detained in the eastern town of Castellon, was close to several members of the group, especially Abdelbaki Es Satty, a Moroccan imam believed to have radicalised the youths that carried out the attacks, it said in a statement.
He is believed to have helped the cell obtain the ingredients they used to make over 100 kilos (220 pounds) of the explosive TATP and provided them with vehicles to transport it.
"In addition, he provided the terrorists with methods of transportation used to move part of the explosives."
TATP has become the explosive of choice for the Islamic State (IS) group as it can easily be made from ingredients such as acetone and oxygenated water that are readily available in high street stores.
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Investigators found 15 kilos of the explosive -- dubbed the "mother of Satan" -- in a flat near Brussels after suicide attacks in the Belgian capital in March 2016.
In last month's attack, jihadists killed 16 people using vehicles and knives in a two-pronged attack in Barcelona and the seaside resort of Cambrils, with the bloodshed claimed by IS.
The main suspects were of Moroccan origin, although most had lived in Spain for several years.
Police shot dead six of the suspected members of the cell that carried out the attacks and arrested another four.
Two other suspects -- including Es Satty -- died in an explosion at a house used by the plotters at Alcanar, southwest of Barcelona, on August 16 on the eve of the Barcelona attack.
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