MADRID - Spanish authorities believe there may have been eight people involved in a cell which carried out an attack in Barcelona on Thursday, and that the group had planned to use butane gas canisters, a judicial source with knowledge of the investigation said on Friday.Catalan government official Joaquim Forn also told local radio earlier on Friday that it was possible that attackers had meant to use canisters in the attack on Thursday in which a suspect drove a van at speed along a busy pedestrian street.Security forces are hunting for the van's driver, who was seen escaping on foot, and police said they had killed five attackers on Thursday night in Cambrils, a town south of Barcelona, to thwart a separate attack."The priority right now is work out the identity of these people, to prove and show the relationship between the different people involved, those that took the van and those that have been able to escape," Forn said. As of Friday morning, three people have been arrested in connection with the attacks, local Catalan police confirmed on Twitter.Catalan emergency services said six civilians and a police officer had been injured in the attack in Cambrils, which police said was linked to Thursday's Barcelona van attack.Catalan police said on Twitter that a bomb squad in Cambrils would carry out several controlled explosions after they determined that the attackers were carrying explosive belts.On Friday morning, the regional head of the region announced that the explosive belts on the suspects were fake.The incident came after a van attack in Barcelona on Thursday that so far has left 14 dead and more than 100 people injured.Spanish Prime Minister Mariano Rajoy said the van attack in Barcelona was "jihadist terrorism" which required a global response."Today the fight against terrorism is the principal priority for free and open societies like ours. It is a global threat and the response has to be global," Rajoy told a news conference in Barcelona.Also on Thursday, hours beforehand, a person was killed in an explosion in a house about 100 km (62 miles) southwest of Barcelona, in an incident linked to the attack, police added.It was still not clear how many attackers had been involved. In another incident, police shot dead a man who had driven a car into a police checkpoint in Barcelona, though they had no evidence that this, too, was connected with the van attack.Witnesses said the white van zigzagged at high speed down Las Ramblas, a busy avenue thronged with tourists, knocking down pedestrians and leaving bodies strewn across the ground.Islamic State's Amaq news agency said: "The perpetrators of the Barcelona attack are soldiers of the Islamic State and carried out the operation in response to calls for targeting coalition states" - a reference to a US-led coalition against the Sunni militant group.The claim could not immediately be verified.