One person was killed and at least seven were injured in an explosion at a house in Catalonia which investigators are linking to the Barcelona attack, police said.
There are suspicions that the blast points to a larger plot which went wrong.
The explosion, which took place on Wednesday night in Alcanar, was reported to have hurt police officers and firefighters.
One of the attack suspects was arrested in the town on Thursday, which lies around 120 miles south of Barcelona.
It is believed the blast, which could be heard for miles around, was caused by butane gas, but the circumstances were unclear.
Around 20 bottles of gas were reportedly found within the ruins of the house, which was totally levelled. Several other nearby houses were also damaged.
Police chief Josep Maria Estela yesterday said the finding the gas was "surprising". It was initially believed the house could have been a drug laboratory.
Analysts said the amount of gas found in the house was too much for normal use, indicating plans for a large bomb attack.
On Twitter, security analyst Aldin Abazovic speculated: "Two rented vans plus this amount of gas bottles, result is a two solid SVBIEDs [Suicide Vehicle-Borne Improvised Explosive Device].
"Just a theory of course."
At least 13 people were killed today and 100 injured when a van rammed into a crowded street in Barcelona.
The driver is believed to have escaped.