The death toll from the attacks rose to 15 on Monday. In addition to the 13 people killed in the van attack in Barcelona, a 14th person died hours later in a related attack in the town of Cambrils, and the police on Monday announced a 15th victim: the driver of the car that Mr. Abouyaaqoub stole and used to make his escape.
The attacks were Spainâs deadliest terrorist assault since 2004, when terrorists bombed commuter trains in Madrid, killing 192 people.
Also on Monday, the authorities said that an imam believed to have inspired the twin attacks had almost certainly died on Wednesday when a house that the terrorists used as a bomb factory blew up â an event that appears to have precipitated the attacks.
The imam, Abdelbaki Essati, preached in the town of Ripoll, home to many of the members of the terrorist cell, which the authorities say included at least 12 people.
Investigators believe the planning for the plot may have begun not long after Mr. Essatiâs arrival, a year ago, at the second of two mosques where he worked in Ripoll.
The remains of two people were found at the house where the explosion took place, in the town of Alcanar, south of Barcelona.
Maj. Josep LluÃs Trapero, the police chief in the Catalonia region, of which Barcelona is the capital, said at a news conference on Monday that the police had âsolid indicationsâ that Mr. Essati was one of them, although they were awaiting the results of DNA tests. The other is yet to be positively identified.
The police also said on Monday that they were certain that Mr. Abouyaaqoub was the driver of the van.
They released surveillance camera images of Mr. Abouyaaqoub, wearing a striped polo shirt, and gave details about how he managed to escape from downtown Barcelona. âWe believe he was the only one in the van and driving it,â Major Trapero said.
Mr. Abouyaaqoub fled on foot from Las Ramblas, the police said, and crossed another popular tourist destination, La Boqueria, a busy food market. He then spotted a stationary car in the cityâs university district, killed the owner and put the body on the back seat. Then he forced his way through a police check point.
The driver, Pau PÃ©rez, was found stabbed to death in his vehicle on the outskirts of Barcelona.
Two of the 15 people killed were children, including a 7-year-old who had Australian and British citizenship. Six victims were Spanish, including one who also held an Argentine passport. Three were Italians, two were Portuguese, one Belgian, one American and one Canadian, the authorities said.
Fifty of the victims remained in hospitals on Monday, 12 of them in critical condition, down from the 126 who were taken to the hospital immediately after the attacks.
The police chief said that the investigation had gained an international dimension, implying that other countriesâ police and intelligence agencies were now involved, but did not provide details.
He also would not comment on reports that the imam had longstanding ties to extremists and had spent time overseas, including in Belgium early last year, shortly before terrorists attacked the airport and subway in Brussels. The imam spent time in prison in Spain on drug-related charges, but had no record for terrorism-related activities.
Major Trapero defended the level of police surveillance ahead of the attacks.
The country has avoided major acts of jihadist terrorism since the Brussels attack, even as the Islamic State and other extremists struck other cities across Europe.
Major Trapero said that it would be âplaying dirtyâ to accuse the police of lapses, and that the police had never received information that would have justified acting against members of the cell.
Asked why nobody had raised the alarm in Alcanar, as terrorists stored over 100 gas cylinders in their bomb-making house, Major Trapero said: âWe have to be cautious not to criminalize the ones who didnât see or act.â
Even with Mr. Abouyaaqoub on the run, Spain kept its level of terrorism alert on Monday at four, on a scale from one to five.
At a news conference in Madrid, Interior Minister Juan Ignacio Zoido praised the âfluidâ cooperation between the authorities in Barcelona and Madrid since the attacks. Mr. Zoido urged citizens to join a march next Saturday in Barcelona to condemn terrorism. âWe all make ours the suffering of Barcelona,â he said.