According to a senior police official, neither of the two detained suspects -- a Spanish national from Melilla and a Moroccan -- is the van driver.
Police Major Josep Lluis Trapero says the third man drove the attack van down the pedestrian walkway from the top of Las Ramblas to near the city's opera house and then set out on foot. Trepero says it doesn't appear the driver was armed.
The Islamic State claimed responsibility for the deadly attack, saying it was carried out by "soldiers of the Islamic State." It says the attack was in response to IS calls for its followers to target countries participating in the coalition trying to drive the extremist group from Syria and Iraq.
The afternoon attack in the northeastern Spanish city was the country's deadliest since 2004, when al-Qaida-inspired bombers killed 192 people in coordinated attacks on Madrid's commuter trains.
Las Ramblas is a large pedestrian thoroughfare in the city that attracts large tourist crowds. Barcelona police, in a tweet, told people to stay away from the center of the city because of a "large contingent of security forces and emergency services" deployed in the area.
Barcelona resident Keith Fleming says he was watching television in his building on a side street just off Las Ramblas when he heard a noise and went out to the balcony to investigate.
He says he saw "women and children just running and they looked terrified." Fleming heard a bang, possibly from someone rolling down a store shutter, as more people raced by.
The American living in the Spanish city says police arrived and pushed everyone a full block down the street.
The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!— Donald J. Trump (@realDonaldTrump) August 17, 2017
Cars, trucks and vans have been the weapon of choice in multiple extremist attacks in Europe in the last year.
The most deadly was the driver of a tractor-trailer who targeted Bastille Day revelers in the southern French city of Nice in July 2016, killing 86 people. In December 2016, 12 people died after a driver used a hijacked truck to drive into a Christmas market in Berlin.
There have been multiple attacks this year in London, where a man in a rented SUV plowed into pedestrians on Westminster Bridge, killing four people before he ran onto the grounds of Parliament and stabbed an unarmed police officer to death in March.
Four other men drove onto the sidewalk of London Bridge, unleashing a rampage with knives that killed eight people in June. Another man also drove into pedestrians leaving a London mosque later in June.
The Associated Press contributed to this report.
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