Police in Catalonia launched an appeal for information about the fugitive, Younes Abouyaaqoub, 22, believed to be the last remaining member of a 12-man cell.
The other suspects have been killed by police or detained following last week's attacks in Barcelona and the seaside resort of Cambrils. The Islamic State (IS) claimed responsibility for the assaults, believed to be its first in Spain.
Authorities today raised the death toll of the attacks to 15, confirming that Pau Perez, a 34-year-old man found stabbed dead in a Ford Focus outside Barcelona on Friday, was killed by Abouyaaqoub.
Investigators believe the victim was the owner of the car, which was hijacked by Abouyaaqoub to make his getaway after the Barcelona assault.
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Describing Abouyaaqoub as around five feet, 11 inches tall, police tweeted four photographs of the man with short black hair, including three pictures in which he was wearing a black and white striped T-shirt.
He is "dangerous and could be armed," police in Catalonia said today, as officials also announce that they have identified all 15 victims in the assaults.
"We have to talk to European police to notify them of the identity because this person... Is likely being sought in all European countries," said Catalonia regional interior minister Joaquim Forn.
Investigators seeking to unravel the terror cell have homed in on the small border town of Ripoll, at the foot of the Pyrenees mountains.
Several of the suspects including Abouyaaqoub grew up or lived in the town.
A Moroccan imam, Abdelbaki Es Satty, aged in his 40s, has also come under scrutiny as he is believed to have radicalised youths in Ripoll.
Police said the imam had spent time in prison and had once been in contact with a suspect wanted on terrorism charges but was never charged with terror related incidents himself.
In Belgium, the mayor of the Vilvorde region told AFP that Satty spent time in the Brussels suburb of Machelen -- a district next to the city's airport -- between January and March 2016.
On the other side of Brussels, the Molenbeek suburb has gained notoriety as a hotbed of international jihadists after the Brussels bombings in March 2016 and the Paris attacks in November 2015.
"Over the last two years, Younes and Houssein began to radicalise under the influence of this imam," their grandfather told AFP.
A neighbour close to the Abouyaaqoub family, speaking on condition of anonymity, said the imam "had recruited Moroccans of Ripoll and planned the attacks."
But Ali Assid, president of the Annour Islamic community that runs the mosque in Ripoll where Satty was preaching, said the imam "never sent a radical message, all he preached was really Islam. If he is behind all that, there he must be showing us one face in the mosque and showing another face outside."
The imam has been missing since Tuesday. On Saturday, police raided his apartment. They have raised the possibility that he died in an explosion on Wednesday evening at a house believed to be the suspects' bomb factory, where police uncovered a massive cache of 120 gas canisters.
The suspected jihadists had been preparing bombs for "one or more attacks in Barcelona", regional police chief Josep Lluis Trapero told reporters, revealing that traces of triacetone triperoxide (TATP) -- a homemade explosive that is an IS hallmark -- had also been found.
Instead, they used a vehicle to smash into crowds on Barcelona's Las Ramblas boulevard as it was thronged with tourists, killing 13 people and injuring about 100.
Several hours later, a similar attack in the seaside town of Cambrils left one woman dead. Police shot and killed the five attackers in Cambrils, some of whom were wearing fake explosive belts and carrying knives.