Aug. 18 (UPI) -- Spanish police have arrested four people believed to be part of a covert, radical Islamic terrorist cell, but were still engaged in an intense manhunt Friday for the driver connected to a deadly van attack in the heart of Barcelona's tourist district.
Police are seeking the driver, identified as Moussa Oukabur Soprano. Four other suspects have been arrested. Five more suspected members of the terrorist cell were killed in shootouts with police and a sixth is believed to have been killed when homemade bombs inside the house where they plotted the attacks exploded Wednesday morning.
The deadliest part of the attack happened in Barcelona's Las Ramblas tourist district, where police say the van drove over a curb into a large crowd, killing 14 and injuring 120 more.
The U.S. State Department confirmed Friday that an American was among the dead.
The search for suspects has become Spain's biggest manhunt in years.
The Islamic State quickly claimed responsibility. The location and targeting of the attack deviates from typical Islamic State terrorism attempts, which focus on punishing countries directly involved in anti-IS military operations in Syria and Iraq. The Islamic State also generally takes responsibility after all suspects are dead or in custody, to prevent information about its operations from being spread.
As the dust settled Friday, it became clear the country had been targeted by a group of terrorists operating across a wide area of the Spanish coast.
The first hint something was amiss came Wednesday, when a house in Alcanar, a coastal town 125 miles south of Barcelona, exploded, killing a Spanish national inside and injuring six others, including another member of the cell who has since been arrested. At the time, police did not suspect terrorism as the cause of the explosion.
Police now believe the accidental detonation of homemade butane bombs forced the perpetrators to alter the plans and timing for their attack. Without the bombs, police said they rented multiple vehicles and planned to use them to mow down pedestrians. A string of similar attacks have happened across Europe in recent months.
One van was used in Las Ramblas, killing 14. A second van was identified and seized in Vic. A third vehicle was used early Friday in an attack on Cambrils, another coastal town 70 miles south of Barcelona, where one pedestrian was killed and the five suspected perpetrators were also killed in a shootout with police. The suspects were also wearing fake suicide vests.
Catalan Police Chief Jose Lluis Trapero said police "are working on the hypothesis that these attacks were being prepared in that house." In using a van instead of explosives, it made the Barcelona attack "more rudimentary than they originally planned," he said.
Spanish Prime Minister Mariano Rajoy said the events were the result of "jihadi terrorism." Other world leaders condemned the attacks Friday and pledged vigilance against terrorist threats.
U.S. President Donald Trump said on Twitter Friday that "Homeland Security and law enforcement are on alert & closely watching for any sign of trouble." Trump added that U.S. "borders are far tougher than ever before!"
He later tweeted, "Radical Islamic Terrorism must be stopped by whatever means necessary! The courts must give us back our protective rights. Have to be tough!"
The International Red Cross set up medical posts at the Barcelona site immediately after the attack. It reported that food and water were distributed to those left stranded and helped to transport people back to homes and hotels. People from 34 countries are among the injured in the Barcelona attack, the Catalan government reported. The area where it occurred, Catalunya Square, is a popular tourist site.