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LONDON — The driver of the van that rammed into crowds on Barcelona's La Rambla, killing 13 people including an American, may have crossed the border into France, Spanish police said Sunday.

“We don't have any specific information on this but it cannot be ruled out," Catalan police chief Josep Lluis Trapero told a news conference in Barcelona.

Trapero said the manhunt continued more than 48 hours after the driver was seen fleeing the scene of the attack on foot.

Police said they thought there was only one person in the van at the time of the attack.

Trapero said police had identified 12 suspects in this week’s twin vehicle attacks in Barcelona and the seaside town of Cambrils which killed a total of 14 people and injuring more than 120. Police said they were still searching for three suspects, while five were dead and four had already been detained.

They believe the suspects were part of a group which had planned one or more attacks on Barcelona using explosives.

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Hours after the downtown van ramming on Thursday, an Audi A3 plowed into pedestrians in Cambrils, 60 miles south of Barcelona, killing one person and injuring six. Minutes later, police shot dead the five attackers who were wearing fake bomb belts.

Four suspects have been arrested, one in Alcanar, southwest of Barcelona, and three in Ripoll.

Younes Abouyaaqoub, one of the suspects. - / AFP - Getty Images

Two people are also believed to have died in a house explosion on Wednesday night in the town of Alcanar, around 100 miles to the southwest. Police believe the property was used to plan the attacks, and that the accidental explosion prevented a far deadlier attack, according to the Associated Press.

Police tweeted Sunday that they are still looking for three people because they had not confirmed the identity of the human remains found in the house. At least one of the three missing suspects remains at large.

Police named one missing suspect Saturday as Moroccan-born Younes Abouyaaqoub, but could not confirm whether the 22-year-old was the driver of the vehicle, or if he had played some other role.

The new details came as police said they could not rule out whether a separate incident on Thursday in the town of Sant Just, on the outskirts of Barcelona, was linked to the two vehicle attacks.

The details of the incident remain unclear but a police spokeswoman said one officer was hurt when a car tried to breach a police checkpoint in the town on Thursday evening.

Spain's King Felipe VI, Spain's Queen Letizia and Portugal's President Marcelo Rebelo de Sousa leave a mass to commemorate victims. PASCAL GUYOT / AFP - Getty Images

The car was later found abandoned and a Spanish man was found dead inside, according to police. The man had been fatally stabbed.

Of the 14 people killed in this week's attacks, nine have been formally identified and three are in the process of identification, police said Sunday. Officers are waiting on the DNA results for two others. Five of the victims are Spanish, two are Italians, two are Portuguese, one Belgian, one Canadian and one American, according to Reuters.

Later on Sunday police confirmed that a missing 7-year-old boy, a dual British-Australian national, was among the dead in the van-attack.

Related: Scared but Defiant, Barcelona Marches to Reclaim City From Terrorists

Meanwhile, Spain’s King Felipe VI and Queen Letizia attended a mass for the victims of the terror attacks at Barcelona’s iconic Sagrada Familia Cathedral.

Barcelona's football team will wear special shirts, bearing the Catalan words for "We are all Barcelona", and black armbands in memory of victims when they play their opening league game of the season on Sunday evening against Real Betis.