Spanish Prime Minister Mariano Rajoy has attended an emergency security meeting in Barcelona to coordinate the investigation into the terror attacks in the northeastern region of Catalonia.
It comes as Spanish radio asked Catalonia's President Carles Puigdemont on Friday whether an alleged "terrorist" was still at large and dangerous to which he replied: "On the run yes, as for whether he is capable of harm, we don't know at the moment."
But he warned that "these types of people have already demonstrated that they have the will to harm whatever happens."
Spain is waking up to a double tragedy after drivers ploughed into pedestrians in two quick-succession, separate attacks in Barcelona and Cambrils, some 120 kilometres south of the seaside city loved by tourists around the globe.
Authorities have confirmed the death toll has risen from 13 to 14, however Australia's Foreign Minister Julie Bishop had earlier reported 16 had died in the immediate aftermath of the attack.
In the first incident, which was claimed by IS, a white van sped into a street packed full of tourists in central Barcelona on Thursday afternoon, knocking people out of the way and killing 13.
Some eight hours later in Cambrils, an Audi A3 car rammed into pedestrians, injuring six civilians - one of them critically - and a police officer.
Spanish emergency authorities said that a woman who had been injured later died.
Spain's security forces have named the driver of the van in Barcelona as Moussa Oukabir, the 17-year-old brother of one of the three people arrested in connection with the crime, local El Mundo newspaper reports, quoting investigative sources.
Oukabir's older brother Driss, was arrested in Ripoll, about 100 kilometres north of Barcelona, police said on Friday.
Investigators believe that the Moroccan teenager stole his brother's ID to rent two vans, one of which was used for the attack.
The second one was found in Vic, about 70 kilometres north of Barcelona, El Mundo said.
Barcelona's players and coaching staff have held a minute's silence before training on Friday in tribute to the victims of the terror attack in the city, in which up to 13 people are feared to have died and dozens more injured.
Barca has already confirmed that players of all their teams would wear black armbands during games this weekend - the senior side begin their LaLiga campaign against Real Betis at the Nou Camp on Sunday.
A minute's silence will be held at matches across Spain following the attack, which happened when a van ploughed into pedestrians in Barcelona's busy tourist area of Las Ramblas.
Barca tweeted a short video of the training tribute, in which players and staff stood in a semi-circle at the club's training ground while holding a minute's silence.
Sixteen people were killed Thursday when a driver deliberately slammed a van into crowds on Barcelona's most popular street in an attack claimed by the IS.
Police and the Catalonian Interior Minister said they had arrested three suspects after the assault, the latest in a wave of vehicle rampages across Europe in recent years.
The IS propaganda agency Amaq claimed that "soldiers" from the jihadist group carried out the attack, according to the Site Intelligence Group which monitors Islamist websites.
Witnesses told of the panic unleashed in an area thronging with local residents and tourists in Spain's second biggest city, as world leaders united in condemning the carnage.
"We can confirm there are 13 dead and more than 50 injured," regional interior minister Joaquim Forn said on Twitter. Belgium said one of its nationals was among the dead.
Twenty-six French citizens have been injured, with at least 11 of them in serious condition, the French Foreign Affairs Ministry says.
The statement says French Foreign Affairs Minister Jean-Yves Le Drian will visit Barcelona on Friday to visit the victims, and that the French consulate in Barcelona is in contact with Spanish authorities.
One of the suspects was named by the police union as Driss Oukabir, but there were no further details and police denied earlier reports a perpetrator was holed up in a bar.
The famous Las Ramblas is one of Barcelona's busiest streets, lined with shops and restaurants and normally packed with with tourists and street performers until well into the night.
Police said there had been a "huge collision" between a van and pedestrians on the thoroughfare and a police source said officers were seeking a total of two suspects.
Spain's royal family condemned the assault in unusually strong terms, vowing that their country would not be "terrorised" by extremists.
Witnesses told of scenes of horror, with bodies strewn along the boulevard as others fleeing for their lives.
"When it happened I ran out and saw the damage," local shop worker Xavi Perez told AFP.
"There were bodies on the ground with people crowding round them. People were crying. There were lots of foreigners."
Witness Aamer Anwar told Britain's Sky News television that he was walking down Las Ramblas, which he described as "jam-packed" with tourists.
"All of a sudden, I just sort of heard a crashing noise and the whole street just started to run, screaming. I saw a woman right next to me screaming for her kids."
Just hours after, police stopped a second terrorist attack involving explosive devices in the Spanish coastal town of Cambril.
Police in the resort town of Cambrils, located about 100 kilometres south of Barcelona, say those behind a van attack which left six people injured were carrying explosive vests, according to the Associated Press.
The four attackers shot dead by police in Cambrils, 120 kilometres south of Barcelona, were wearing explosive belts.
Police were preparing to complete controlled explosions and called for calm in Cambrils ahead of the operation.
Spain had so far been spared the kind of extremist violence that rocked nearby France, Belgium and Germany.
But it was hit by what is still Europe's deadliest jihadist attack in March 2004, when bombs exploded on commuter trains in Madrid, killing 191 people in an attack claimed by Al Qaeda-inspired extremists.
Ethan Spibey, a charity director on holiday in the city, said he and several others had locked themselves in a nearby church.
"All of a sudden it was real kind of chaos... people just started running screaming," he told Sky. "There was kind of a mini stampede."
Tom Gueller, who lives on a road next to Las Ramblas said he saw the van speeding along the boulevard.
"It wasn't slowing down at all. It was just going straight through the middle of the crowds in the middle of the Ramblas," he told BBC radio.
A Greek diplomat in the city said three nationals had been wounded - a woman and her two children.
Thursday's attack, which followed similar incidents in Britain, Germany and France, drew widespread condemnation.
"The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help," US President Donald Trump tweeted.
France's President Emmanuel Macron - whose country has witnessed a series of bloody jihadist atrocities including a truck rampage in Nice in July 2016 that killed 86 people - said his thoughts were with the victims of the "tragic attack".
A spokesman for German Chancellor Angela Merkel condemned the "revolting attack" and British Prime Minister Theresa May said on Twitter that London "stands with Spain against terror".
The Nice carnage and other assaults including the 2015 Paris attacks on nightspots in the city were claimed by the Islamic State.
In another deadly vehicle attack in December, 12 people were killed when a man driving a truck ploughed into a crowd at a Berlin Christmas market.
Spain has emerged as a potential target for jihadists, with extremist websites mentioning it for historical reasons, since much of its territory was once under Muslim rule.
The authorities in Spain - the world's third biggest tourism destination - generally remain discreet on the terror threat.
But they publicise every arrest of alleged jihadists, most of them detained for propaganda, recruitment for extremist groups or "glorifying terrorism."
According to the interior ministry, more than 180 "jihadist terrorists" have been arrested since 2015.