BARCELONA, Spain -- Police put up scores of roadblocks across northeast Spain on Sunday in hopes of capturing a fugitive suspect from the 12-member Islamic extremist cell that staged two vehicle attacks and plotted much deadlier carnage using explosives favored by Islamic State of Iraq and Syria (ISIS) militants.

Complicating the manhunt, though, was the fact that police have so far been unable to officially identify who exactly is at large.

While police have identified the 12 members of the cell, three people remain unaccounted for: two believed killed when the house where the plot was being hatched exploded Wednesday, and a suspected fugitive, Catalan police official Josep Lluis Trapero told reporters Sunday.

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Trapero declined to confirm that Younes Abouyaaquoub, a 22-year-old Moroccan, was the one at large and the suspected driver of the van that plowed down Barcelona's Las Ramblas promenade Thursday, killing 13 people and injuring 120. Another attack hours later killed one person and injured others in seaside town of Cambrils.

"We are working in that line," Trapero said. But he added: "We don't know where he is."

Another police official did confirm that three vans tied to the investigation were rented with Abouyaaquoub's credit card: The one used in the Las Ramblas carnage, another found in the northeastern town of Ripoll, where all the main attack suspects lived, and a third found in Vic, on the road between the two.

Sources told CBS News the terrorists were planning a larger attack. Police believe the cell members had planned to fill the vans with explosives and create a massive attack in the Catalan capital. Trapero confirmed that more than 100 tanks of butane gas were found at the Alcanar house that exploded, as well as ingredients of the explosive TATP, which was used by ISIS in attacks in Paris and Brussels.

"Our thesis is that the group had planned one or more attacks with explosives in the city of Barcelona," he said. That plot was foiled, however, when the house in Alcanar blew up Wednesday night. It was leveled, and the explosions were so strong that a neighbor showed CBS News how it broke the glass in her window. Spanish police theorize that after the explosives prematurely ignited, the terrorists went to plan B.

Spain's interior minister said Saturday the cell had been dismantled.

The investigation is also focusing on a missing imam, Abdelbaki Es Satty, who police think could have died in the Alcanar explosion. Trapero confirmed the imam was part of the investigation but said police had no solid evidence that he was responsible for radicalizing the young men in the cell.

Es Satty in June abruptly quit working at a mosque in Ripoll and has not been seen since.

His former mosque denounced the deadly attacks and weeping relatives marched into a Ripoll square on Saturday, tearfully denying any knowledge of the radical plans of their sons and brothers. Abouyaaquoub's mother said his younger brother Hussein has also disappeared, as has the younger brother of one of five radicals shot dead Friday by police during the Cambrils attack.

Everyone so far known in the cell grew up in Ripoll, a town in the Catalan foothills 100 kilometers (62 miles) north of Barcelona. Spanish police searched nine homes in Ripoll, including Es Satty's, and set up roadblocks. French police carried out extra border checks on people coming in from Spain.

Neighbors, family and the mayor of Ripoll said they were shocked by news of the alleged involvement of the young men, whom all described as integrated Spanish and Catalan speakers.

Halima Hychami, the weeping mother of Mohamed Hychami, an attacker believed to have been killed by police, said he told her he was leaving on vacation and would return in about a week. His younger brother, Omar, left mid-afternoon Thursday and has not been heard from since.

"We found out by watching TV, same as all of you. They never talked about the imam. They were normal boys. They took care of me, booked my flight when I went on vacation. They all had jobs. They didn't steal. Never had a problem with me or anybody else. I can't understand it," she said.

Fatima Abouyaaquoub, sister-in-law of the Hychami brothers and the cousin of Younes Abouyaaquoub, found it all hard to believe.

"I'm still waiting for all of it to be a lie. I don't know if they were brainwashed or they gave them some type of medication or what. I can't explain it," she said.

Islamic extremists have made a point of targeting Europe's major tourist attractions -- especially in rented or hijacked vehicles. But in the last two years, the extremist group has steadily lost ground in its self-declared caliphate in Iraq and Syria.

Portuguese president Marcelo Rebelo de Sousa, King Felipe of Spain and his wife Letizia are seen leaving after the High mass in the Basilica of the Sagrada Familia in memory of the victims of the van attack at Las Ramblas in Barcelona earlier this week, Spain August 20, 2017. Reuters

On Sunday, King Felipe VI, Queen Letizia, Spanish Prime Minister Mariano Rajoy and other officials attended a solemn Mass at Barcelona's Sagrada Familia basilica for the victims of the attacks.

The archbishop of Barcelona, Cardinal Joan Josep Omella, said the presence of so many people was a "beautiful mosaic" of unity and urged all to work toward a common objective of "peace, respect, fraternal coexistence and love."

The 14 people killed spanned generations -- from age 3 to 80 -- and left behind devastated loved ones.

Vancouver police said Sunday that 53-year-old Canadian Ian Moore Wilson was among those killed in Barcelona and his wife Valerie was wounded.

Fiona Wilson, his daughter, described her father as an adventurous traveler and thanked those who helped him and her mother in his final moments.

"In the midst of this tragedy, my dad would want those around him to focus on the extraordinary acts of human kindness that our family has experienced over the past several days, and that is exactly what we intend to do," she said.

A police van blocks the the street at the beginning of fiestas, after a van ran down pedestrians in Barcelona's Las Ramblas on Thursday, in Bilbao, Spain, on Sat., Aug. 19, 2017. Reuters

Francisco Lopez Rodriguez, a 57-year-old Spaniard, was killed with his 3-year-old grand-nephew, Javier Martinez, while walking along the Las Ramblas promenade. His widow Roser is recovering in a hospital.

"We are a broken family," niece Raquel Baron Lopez posted on Twitter.

Other victims included a grandmother, 74, and her granddaughter, 20, from Portugal who were visiting Barcelona to celebrate a birthday; an Italian father who saved his children's lives but lost his own; an American man who was celebrating his first wedding anniversary.

By late Saturday, the Catalan emergency service said 53 attack victims were still hospitalized, 13 of them in critical condition.