The 18-year-old man being sought in the deadly attack in Barcelona fantasized online about killing the “unbelievers” and leaving “only Muslims who follow their religion,” according to a report.

More details have emerged about Moussa Oukabir, the younger brother of Driss Oukabir, who was arrested Thursday on suspicion of renting the white van used in the attack at Las Ramblas.

When asked by the social media site Kiwi what he would do if he became the absolute world leader, he replied, “Kill the unbelievers and leave only Muslims who follow their religion,” according to the UK’s Guardian.

Asked in which country he would never consider living, he answered: “The Vatican.”

Catalonian government sources told the Telegraph of the UK that he spent a month in custody in Figueres prison, suspected of sexual abuse, before being released in 2012.

Driss, a Moroccan who was nabbed in Ripoll, about 60 miles north of Barcelona, has denied involvement and told police that his identity documents were stolen before they were used to secure the vehicle.

Catalan authorities believe his brother fled the scene of the carnage, though the Mossos d’Esquadra police force was not officially naming suspects, a spokeswoman told the Guardian of the UK.

“We are looking for Moussa Oukabir, but for the moment we don’t know whether he was the driver of the van,” police said.

Catalonian Interior Minister Joaquim Forn said the “top priority” is to find the driver of the vehicle that left 13 people dead and scores injured in Barcelona.

“We don’t know if the driver is still in Barcelona or not, or what direction he fled in,” Forn told a local radio, the Wall Street Journal reported. “We had local police on the scene, but we were unable to shoot him, as the Ramblas were packed with people.”

A fourth person was arrested Friday in connection with the attack in Barcelona and another incident in the seaside town of Cambrils, where one woman lost her life after an Audi A3 barreled into tourists on the Costa Dorada.

Seven people including a cop were injured in the attack, which was being linked by Spanish authorities to the strike in Barcelona.

Five suspected jihadists wearing fake suicide vests were shot dead by police as they jumped out of their car, clutching knives and taunting cops.

A British man who saw one of the attackers being shot by police told the BBC that he had seen people running into the bar where he was as the assault unfolded.

“This guy came running up the road and was shouting something. I didn’t know what it was, so we said call the police,” said Fitzroy Davies, of Wolverhampton, the Guardian reported.

“Within 30 seconds the police was [sic] already there, jumped out of the car, started shouting at the guy; the guy was then saying something else again,” he said. “And then they — ‘pop, pop’ — did a couple of shots and he fell down.

“He stood back up and then he stepped over the fence and he started, he was taunting, smiling and he carried on walking to the police, and then they gave it to him again, a couple more shots and then he fell to the ground.”

ISIS claimed responsibility for the Barcelona bloodbath via its official Amaq news agency, according to the SITE Intelligence Group, which monitors jihadist activity online.

Ripoll Mayor Jordi Munell told Catalonia’s TV3 channel that the Oukabirs came from a “normal family,” the Telegraph reported.

“We are surprised to see these tragic events being related to a family from Ripoll,” he said. “They have lived here for many years and we have never had a problem. People here are sad and worried.

“We are a town of around 11,000 people with 9 percent immigrants. We practically all know each other,” he added.

With Post wires