THE family bonds of trust amongst brothers are creating new obstacles for intelligence services seeking to infiltrate and disrupt terror cells linked to Islamic State intent on bringing murder and mayhem to European capitals.

Terrorist groups are deliberately recruiting families - especially brothers - to ensure that attacks are carried out successfully and in utmost secrecy, intelligence experts have told the Sunday Herald.

Two of the suicide bombers who carried out attacks in Brussels that killed 34 and injured more than 270 on Tuesday were named as brothers Khalid and Brahim el-Bakraoui.

It is the latest example of how terrorist groups are exploiting family bonds to create the covert and clandestine cells bound not only by ideology but also by blood. These deep family attachments mean related extremists can share complete devotion to, and secrecy about, planned attacks.

It is understood the brothers who attacked Brussels airport were listed in US databases as potential terror threats.

Belgium's interior minister Jan Jambon and Koen Geens, the justice minister, offered their resignations over security failures surrounding Ibrahim el-Bakraoui who was deported from Turkey to the Netherlands in July, last year amid concerns that he was a militant but was not being monitored by Belgian intelligence.

The trail of so-called brothers-in-arms involvement in terror attacks goes back as far as the 9/11 attacks, 15 years ago.

HeraldScotland:

Both American Airlines Flight 11 piloted by Mohammed Atta into the World Trade Center and Flight 77 piloted by Hani Hanjour into the Pentagon, involved sets of brothers. Wail and Waleed al-Shehri and Nawaf and Salem al-Hazmi served as the 'muscle' needed to take over the aircraft.

Brothers Ibrahim Abdeslam and Salah Abdeslam - the latter now in custody - were both reportedly involved in the November Paris attacks and seem to have become radicalized together.

HeraldScotland:

Ibrahim Abdeslam was the suicide bomber who blew himself up outside the Comptoir Voltaire restaurant – while his brother, who appeared to have been principally a logistician and organizer was on the run until March 18, when he was captured in Brussels, and charged with terrorist murder.

A third brother, Mohammed, was arrested in Brussels on Monday and spent several hours in custody before being released without charge.

Two other brothers Cherif and Said Kouachi entered the offices of the French satirical newspaper Charlie Hebdo in January 2015 and gunned down 12 people in and around its offices, including eight of its journalists and cartoonists.

HeraldScotland:

Chérif Kouachi (left) and Saïd Kouachi (right)

Cherif, 32, appeared to be the more hardline of the pair, and to have influenced his brother.

The brothers of Algerian descent, who were orphaned as children, were reportedly radicalized as they began attending the Adda'wa mosque in Paris in 2003.

They were killed two days after the attack when police closed in on their hideout north-east of Paris.

Brothers Tamerlan and Dzhokhar Tsarnaev also carried out the 2013 bombings at the Boston Marathon, which killed three people and injured more than 260.

HeraldScotland:

Tamerlan Tsarnaev (left) and younger brother Dzhokhar Tsarnaev were ethnic Chechen Muslims

Tamerlan, 26, died of gunshot and blast wounds at a Boston hospital on April 19 after a firefight with police in Watertown, just outside of Boston. Dzhokhar, then 19, was found that same night, hiding in a dry-docked boat behind a home.

The brothers had detonated two backpacks with household pressure cookers loaded with explosives, nails and ball bearings at the marathon. Dzhokhar was sentenced to death by execution in June 2015.

Mohammed and Abdelkader Merah were linked to the 2012 terror attacks in Toulouse and Montauban in France.

HeraldScotland:

Mohammed (above), 23, was the gunman in the attacks which killed seven people from March 11 to 19 before he was killed by police on March 22. His 29-year-old brother, who was under surveillance for links to radical Islamist groups, is suspected of having indoctrinated his sibling.

Ali Ghufron and younger brothers Amrozi and Ali Imron were involved in the 2002 bombings in Bali, Indonesia, that killed 202 people.

HeraldScotland:

Ali Ghufron (left)and brothers Amrozi and Ali Imron

Ali Ghufron was the operational chief of Jemaah Islamiyah, a militant Islamist group in Southeast Asia. He is said to have recruited his two brothers.

In 2003, Ali Imron was sentenced to life in jail while Amrozi and Ali Ghufron were executed by firing squad in 2008.

Tricia Bacon, a former US State Department intelligence analyst said that from an organisational standpoint it makes sense to recruit family members as kinship ties make infiltration far more difficult.

"In addition, brothers can help one another to sustain their commitment when they have doubts," said Bacon, who is now a public affairs lecturer.

"In general, terrorist groups encourage family bonds through existing family ties or inter-marriage to solidify the ties and secure the commitment to the group/cause.

"Therefore, it will be particularly difficult for security services to infiltrate such tight-knit circles, which will necessitate other ways to tap into these networks to gain intelligence.

"Officials would need someone close to the family to inform authorities of the brothers' activities and then an investigation or intelligence collection in order to identify their plans and the broader network, if there is one. In some of the prior cases, other family members and friends were well aware of the brothers' radicalisation."

Research by New America, a non-partisan thinktank in the United States, revealed that one in three western fighters have a familial connection to jihad, whether through relatives currently fighting in Syria or Iraq, marriage, or some other link to jihadists from prior conflicts or terrorist attacks.

The research also found that of those western fighters with familial ties to jihad, three-fifths had a relative who has also left for Syria.

Another study, at Pennsylvania State University, examined the interactions of 120 supposed “lone wolf” terrorists from all ideological and faith backgrounds, and found that, even though they launched their attacks alone, in a large majority of the cases others were aware of the individual’s commitment to a specific extremist ideology. In 64 per cent of cases, family and friends were aware of the individual’s intent to engage in a terrorism-related activity because the offender verbally told them.

Jeff Gardner, a retired US Army lieutenant colonel who became an assistant professor of homeland security studies at the American Military University, said terror groups were exploiting kinship because the bonds are forged far deeper in Islamic communities than in the west.

"These brotherly influences are nothing like in western culture and are also exponentially greater than what a school friend or neighbour would have," he said. "It goes without saying that the brothers are all exposed to the same conditions that brought on radicalisation and subsequent violence, but the bonds they have as family members can, and appear to in these cases, become an irresistible force."

Former US Army intelligence officer Erik Kleinsmith says family members are easier to recruit and drew parallels with the US Army where soliders in basic training buddy-up.

However, unlike many of his peers he also believes the family links can actually help agencies in tracking terror cells, when there is already reliable intelligence.

"It is important to have someone to watch out for the other guy, both physically and mentally. It's easier to go through that process with someone else that you know. Identifying brothers is an advantage both in the training aspect and for bonding them within the operation, because they will move and work together", said Mr Kleinsmith who is now strategic intelligence relationships vice president at the American Military University.

"These operative cells, as they prepare for their missions, as they do their reconnaissance and get themselves mentally ready for attack, they have to have a bond, a life or death bond with other members of the cell, very similar to military units. Men who have been in combat together form lifelong bonds but with family there is already an established bond.

"So operative cells using brothers or sisters are much easier to form and maintain. When you already have these life or death bonds in members of your cell, brothers are willing to endure and sacrifice more than they would if they were operating alone.

Kleinsmith says analysts work to identify friends, family, romantic partners, superiors, underlings, supporters, sympathizers, classmates, or colleagues as part of an endless search for connections between terror group members.

And the appearance of brothers and other family members within an organisation can provide counter-terrorism analysts with potential patterns and trends for further analysis.

Radicalisation is said to be more acceptable to a new recruit once a family member is already involved in the process.

"In an intelligence operation, you can't break the bonds, but what you can do is identify them more easily, because two people have a larger signature than a lone wolf operating by themselves," he said.

"Because they are operating together, it may be tougher to get information, but once you find a pair of brothers, there are some distinct advantages. If you have a pair of brothers you know about, you have a large portion of a cell identified."