As is so often the case in terrorist cells, the group that brought carnage to north-east Spain last week – killing 15 people and injuring more than 130 – included several sets of brothers.
Younes Abouyaaqoub, 22, has been identified by police as the driver of the van that sped down Las Ramblas on Thursday. He was shot dead by police in Subirats, a town 30 miles west of Barcelona, on Monday. His brother Housainne, 19, was among the five men shot dead by police in the coastal town of Cambrils hours later after the car in which they were travelling ploughed into a group of pedestrians.
Mohamed Hychami, 24, and his brother Omar, 21, were shot by police in the same incident.
Mohammed Aallaa, 27, is being questioned by police after his car was used in that attack. Mohammed’s brother Said, 18, was also shot dead in Cambrils. The whereabouts of a third brother, Youssef, remains unknown, leading to speculation that he may have died in a premature detonation that ripped through the cell’s bomb factory in Alcanar on Wednesday night.
Moussa Oubakir, 17, was the fifth terrorist shot dead by police in Cambrils. His brother Driss, 27, is being questioned by police after turning himself in, though there is no confirmation of his involvement in the cell..
While the number of familial connections may appear extraordinary, a number of studies of jihadist radicalisation suggest it is not. Terrorism is a highly social form of activism, and peer influence is a key factor: individuals become interested in ideas, and then committed to a course of action, because others have had the same ideas and developed the same commitment.
Research conducted by a US thinktank, New America, two years ago suggested more than a quarter of individuals who had travelled to Syria or Iraq to fight with Islamic State had a family or marriage connection to another jihadist.
Examples include Abdullah Deghayes, 18, and his brother Jaffar, 17, from Brighton, who both travelled to Syria in 2014 and died in fighting a few months apart.
A study by Pennsylvania State University of 120 supposed “lone wolf” attackers found that in 64% of cases, some family members or friends had been aware of their intentions.
Terrorist brothers include Saïd Kouachi, 34, and his brother Chérif, who attacked the offices of the satirical magazine Charlie Hebdo in Paris in January 2015, opening fire with Kalashnikovs, killing 12, including a Muslim policeman on the street outside.
Brothers Tamerlan and Dzhokhar Tsarnaev bombed the Boston marathon in April 2013, killing three people and injuring several hundred others.
Salah and Ibrahim Abdeslam, who were both involved in the Paris attacks of November 2015, seem to have become radicalised together while growing up in the Molenbeek area of Brussels.
Waleed and Wail al-Shehri sat side by side on the American Airlines aircraft that was flown into the World Trade Centre’s North Tower on 9/11, and are believed to have stabbed two unarmed flight attendants during the hijacking. Nawaf and Salem al-Hazmi were among the hijackers who flew another American Airlines aircraft into the Pentagon on the same day.