MADRID/BARCELONA (REUTERS) - Investigators believe that Younes Abouyaaqoub, a 22-year-old Moroccan-born man sought by police, was the driver of a van that rammed into crowds last Thursday in Barcelona, killing 13 people, a Catalan government official said on Monday (Aug 21).

The hunt for Abouyaaqoub has been extended to other European countries, the official, Joaquim Forn, added.

Spanish police have been searching for Abouyaaqoub - the only one of 12 suspects still at large - for several days but authorities had yet to identify him as the man behind the wheel of the van.

Spanish police have said they could not rule out that he had slipped over the border into France.

Asked on Catalan radio on Monday whether Abouyaaqoub was the driver, Forn, who runs home affairs in Catalonia's regional government said: "This is one of the lines of enquiry. It's the main one. Everything points to that. Today we will explain the evidence and (explain) why we have come to that conclusion."

"This person is no longer just being sought in Catalonia but in all European countries, this is an effort by European police."

Police carried out more raids overnight at homes in the town of Ripoll, where many of the suspects lived, Forn said.

Others thought to be part of a 12-strong cell have been arrested, shot by police or killed in an explosion at a house in Catalonia a day before Thursday's van attack.

In last Thursday's attack, a van was driven through crowds of tourists and locals walking along Las Ramblas, leaving a trail of dead and 120 injured.

A seven-year-old British-Australian boy, Julian Cadman, was confirmed on Sunday as one of 13 killed in the attack.

Hours after the Barcelona attack, police shot dead five men wearing fake explosive belts in the resort of Cambrils, further down the coast, after they rammed holidaymakers with a car and stabbed others, killing one woman.

Four people have been arrested so far in connection with the attacks - three Moroccans and a citizen of Spain's North African enclave of Melilla.

Islamic State in Iraq and Syria (ISIS) said the perpetrators had been responding to its call to target countries involved in a US-led coalition against the Sunni militant group.

In little more than a year, Islamist militants have used vehicles as weapons to kill nearly 130 people in France, Germany, Britain, Sweden and Spain.

Thursday's attacks were the deadliest in Spain for more than a decade and one focus of the Catalan police investigation is how members of the group were radicalised.

One line of enquiry is Ripoll, a quiet town set beneath the Pyrenees mountains, which was home of a number of the mostly Moroccan youths suspected of involvement. Three of those killed in Cambrils were aged between 17 and 24.

Another strand running through the young men's lives was a local imam, Abdelbaki Es Satty, whose landlord said he left Ripoll two days before the attack.

Papers bearing what appeared to be French names, phone numbers and e-mail addresses were seen by reporters in Es Satty's apartment in Ripoll after it was searched by police.

Spanish media have depicted Es Satty as being a possible ring-leader who indoctrinated the group.

Speaking to reporters at the Ripoll mosque on Saturday, the head of the town's Islamic association, Ali Yassine, declined to comment on whether Es Satty had radicalised the suspects and said there were no problems at the mosque when he was imam.

Hannou Ghanimi, the mother of on the run suspect Abouyaaqoub, told reporters on Saturday she wanted her son to give himself up to police, saying she would rather see him in prison than end up dead.

Police believe the group opted to launch attacks using vehicles when their base in Alcanar, southwest of Barcelona, was destroyed in an explosion.

Police believe that foiled the cell's plans to carry out one or more large-scale bombings in Barcelona. More than 100 butane gas cylinders were found in the remains of the Alcanar house.

Spanish media said traces of triacetone triperoxide, a highly volatile explosive that can be made with easily obtainable household chemicals, were also found at the house.

Earlier on Sunday Spain's King Felipe and Queen Letizia, Prime Minister Mariano Rajoy and Portuguese Prime Minister Antonio Costa joined Catalan leaders for a memorial service to the victims.

Mourners have left hundreds of candles, wreaths and bouquets of flowers on Las Ramblas as a tribute. The city's football club, Barcelona FC, meanwhile increased security for its opening league match of the season late on Sunday.

Police armed with assault rifles patrolled before the evening game against Real Betis after a government call for reinforced security at big public events.

"It shows that despite what happened the match has to go on," said Barcelona fan Fakih Hussein. "It's probably even safer than usual now with so much security," said Hussein, 47, who had come with his teenage son.

Barcelona's team will wear black arm bands and special shirts with "We are all Barcelona" in the Catalan language in memory of victims.