After a van sped through a boulevard packed with tourists in central Barcelona on Thursday afternoon, fingers were quickly pointed at Driss Oukabir, a man of Moroccan background in his late 20s who, it appeared, had hired the vehicle.
But it then emerged that his younger brother, Moussa Oukabir, aged 17, may have used his brother's ID to rent two vans in his name: one that killed 13 people and injured more than 100 on Las Ramblas boulevard, and one that was meant as a getaway car.
He was killed by police when he and four other men drove at pedestrians and police in the resort town of Cambrils south of Barcelona. They were wearing fake explosive belts, and had an axe and knives in their car.
But late on Friday, police said their theory that Moussa Oukabir had been the driver now had "less weight".
Instead, the focus of attention turned to Moroccan-born Younes Abouyaaqoub, 22, from the town of Ripoll, where several people had been arrested.
Spanish media say that Abouyaaqoub, who remains at large, is now the main suspect.
Driss Oukabir apparently told police he was not involved and that his documents had been stolen. Police launched a manhunt to find his younger brother, who was suspected to be the driver of the van.
Moussa Oukabir was a Spanish citizen from the northern Catalan city of Girona.
Dubbed "Moussa De La Vega" on Facebook, he appears to have expressed radical sentiments in the past.
Two years ago, he was for a brief period a keen user of a social network call Kiwi, that allows people to ask and answer questions.
Many of his answers were innocent and revealed him to be a boy of few words. "What language would you like to master?" one user asked him. "German", he replied.
Asked what kind of girl he liked, he responded "stylish brunette". But there were also darker answers.
'Kill all infidels'
Someone asked what to do when learning someone has been lying to you for more than a year. Moussa replied: "Kill him at night with a pistol" but then stated that he was only joking and added a crying-with-laughter emoji.
And finally, mixed in among jokes and inane chatter as he passed time with his online friends, he responded bluntly to a question on what you would do on your first day as the supreme king or queen of the world.
"Kill the infidels; leave only Muslims who follow the religion."
The statement certainly suggests radical influences as far back as two years ago, but a young woman said to be close to Moussa Oukabir told LaVanguardia newspaper she was "very surprised by all this", calling him a "very normal and nice guy".
She said while Moussa was on the quiet side, he "never looked for problems".
The teenager appears to have grown up in Spain. On his Facebook page, which has now been deleted, he said he lived in Ripoll, where his brother and one other person were arrested.
The mayor of Ripoll, Jordi Munell, told COPE radio that Driss Oukabir had lived in the town since 1989, and that residents were sad to hear the tragic events in Barcelona may have a connection to Ripoll.
He said that about 9% of the population of 11,000 are immigrants, but "there is no problem of co-existence".
"We almost all know each other," he said.
The town is about 100km (60 miles) north of Barcelona.