Authorities in Spain hunting for four suspected terrorists in widening plot

A manhunt across Spain is underway today for four young men suspected of being part of an ISIS-inspired terror cell that carried out at least two attacks over a 24-hour period, including the van attack in Barcelona that killed at least 14 people.

According to a bulletin issued by European counterintelligence officials obtained by ABC News, authorities are hunting for these suspected terrorists: Moussa Oukabir, 17, Said Aalla, 18, Mohamed Hychami, 24, and Younes Abouyaaqoub, 22 - all Moroccan nationals who live in Spain.

Officials say the van was rented in the name of the older brother of one of the four fugitives, 28-year old Driss Oukabir. He was taken into custody and then released, police say, after they determined his brother Moussa had likely used his identification papers. American law enforcement officials briefed by the Spaniards say a person strongly resembling Moussa appears to be the driver of deadly van attack.

This does not appear to have been a lone wolf attack but rather a much more complex operation involving several people who ISIS claims were soldiers of the caliphate. According to the SITE Intelligence Group, ISIS took responsibility for the Barcelona attack, calling those involved "soldiers of the Islamic state" in a statement released by its 'Amaq News Agency.

Overnight, officials said police foiled a second attempted terror attack, killing five suspected terrorists wearing fake explosive belts in Cambrils who authorities believe were linked to the van attack earlier in the day.

According to a U.S. law enforcement official, an explosion at a house in Alcanar on Wednesday was also connected to Thursday's attack. Authorities discovered more than a dozen propane tanks in the house, suggesting the plotters could have been interested in launching a more sophisticated attack but later settled for something simpler.

The existence and the size of what is now believed to be an ISIS-inspired terror cell comprised of as many as 12 men has alarmed U.S. and European counterterrorism officials.

"ISIS promotes these types of attacks because they're easy," said John Cohen, the former counterterrorism coordinator at the Department of Homeland Security an an ABC News contributor. "You can take people who have received no training, who are using items that are easy to acquire. And they can still commit mass murder."

With five suspected terrorists dead and at least four more in custody, the question now is how many more members of this possible ISIS-inspired sleeper cell are still at large, and will they attack before police can close in.

Two sources in direct contact with US counterterrorism officials say the events in Spain over the last 24 hours make them fear that this is only the beginning of a series of ISIS attacks across Europe.

Officials were concerned about a major series of attacks at the start of the summer and were surprised when things became fairly quiet. They fear the quiet may be over. The belief is that various potential attackers are not in contact with each other but will see the attacks in Spain as the sign to get moving.

Thursday's attack is the just the latest in a string of deadly incidents throughout Europe in which terrorists have drawn from the ISIS playbook of using vehicles to inflict casualties.

In August of last year, 86 people were killed on by a speeding truck driven by an ISIS follower in Nice, France. In December, 12 people killed in a vehicle attack on the Christmas markets in Berlin. And 14 people were killed in two separate incidents in London this year, one on the Westminster Bridge and the other on the London Bridge.

"We have seen a dramatic increase in attacks by individuals who are inspired by ISIS or are loosely connected to ISIS in Europe and even in the U.S.," Cohen told ABC News. "As ISIS suffers more defeats in Iraq and Syria we can only expect to see more attacks."

ABC News' Pete Madden, Randy Kreider and Margaret Katcher contributed to this report.