In the years before his death, Osama bin Laden spent his days behind the walls of his compound in Pakistan, fretting about his son living thousands of miles away.

He penned letter after letter, describing the curriculum that the son, Hamza bin Laden, then 23, should study, the qualities he should cultivate and the safety measures he should follow.

In one, he advised his son, who was just 13 when he saw his father for the last time, not to leave his house.

In another, he discussed whether the young man could rejoin him in Pakistan, advising him to travel on a cloudy day when it would be harder for a drone to track him.

He devised a complicated security protocol, calling for the son to switch cars inside a tunnel in order to fool overhead surveillance.

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The care he showed was not just that of a father for a son. It appears to have also been an attempt by the world’s most hunted terrorist to secure his legacy.

Analysts believe that since at least 2010, al-Qaeda was secretly grooming Hamza bin Laden to take over the organisation, a move that appears to have been foiled.

According to three US officials, the younger bin Laden was killed during the first two years of the Trump administration, though many questions remain, including when, how and where he was killed, and by whom.

If confirmed, his death represents another blow to al-Qaeda, whose ranks were hollowed out by relentless American attacks and by the rise of Isis.

The older terrorist network has struggled to appeal to a younger generation of recruits, who were lured to Isis by slick videos shot on drones and GoPros when al-Qaeda was still issuing hour-long lectures by ageing leaders staring at camcorders.

The younger bin Laden was supposed to solve several of al-Qaeda’s most pressing management issues: No older than 30, he was almost four decades younger than Ayman al-Zawahri, the group’s current leader, who has been vilified by Isis as an old-fashioned and out-of-touch manager.

Because he carries the most famous name in terrorism, the younger bin Laden is able to draw on the devotion that jihadis around the world continue to feel for his father.

For these reasons, al-Qaeda hoped Hamza bin Laden could act as a unifier, appealing not just to the group’s base but also to the recruits it lost to Isis, many of whom are at a crossroads after the loss of Isis’ territory in Iraq and Syria.

Osama Bin Laden lured his son into terror networks the year before he was killed in a US operation in 2011 (AFP/Getty)

“If it’s true that he is dead, then al-Qaeda has lost its future because Hamza was the future of al-Qaeda,” said the former FBI agent and counterterrorism expert Ali Soufan, who sounded a note of caution because it is unusual for al-Qaeda not to announce such a death.

“He was being prepared to lead the organisation, and it’s very obvious from his statements that his focus was to bring back his dad’s message,” said Mr Soufan, who is the author of a profile of Hamza bin Laden calling him “al-Qaeda’s leader in waiting”.

The United States government played a role in the operation to kill him, the US officials said, but they refused to provide any further information. Asked about the operation on Wednesday, Donald Trump, the US president, had no comment.

As for bin Laden’s whereabouts, there were only vague reports about possible sightings.

“Our intelligence reports showed there was a Hamza here, but we didn’t know for sure,” said Mohammad Ismail, the governor of Want Waigal, a mountainous district in eastern Afghanistan near the Pakistani border. “Some would say he was a Pakistani and some would say he was an Arab.”

Letters to and from his father — found by the Navy Seal team that killed the elder bin Laden and later declassified — indicate that he was living in Iran in 2009 and 2010.

One Iranian official said Wednesday that he thought Hamza bin Laden had been living in an upscale villa in Tehran with two wives and a sister. Another official said he came and went but never lived in Iran.

Hamza bin Laden was only 13 when his father walked him and his brothers to the base of a mountain in Afghanistan and said goodbye for the last time. It was 2001 and planes piloted by operatives had just slammed into the World Trade Center and the Pentagon, and another hijacked plane had been foiled on its way to Washington.

Bin Laden documents released

The terrorist leader knew retaliation was not far behind and made arrangements to send his boys away. He handed each of them a set of Muslim prayer beads, reminding them to seek strength in their faith.

“You bid us farewell and we left, and it was as if we pulled out our livers and left them there,” Hamza bin Laden wrote in a letter addressed to “my beloved father” years later.

The identical gifts to his sons suggest the senior bin Laden intended to be equal in his affection. But chroniclers of the family say that it was not long before it became clear that he had a special relationship with Hamza, the only son of Khairia Sabar, a highly educated Saudi woman who became Osama bin Laden’s favourite wife.

When Hamza was two, his father moved from Afghanistan to Sudan. He was there until the age of seven, when the Sudanese government gave in to international pressure and expelled the family.

Osama bin Laden and his followers returned to Afghanistan where they sought refuge with the Taliban and lived in a complex of concrete huts, lacking plumbing, electricity and even doors.

After the 2001 attacks, Hazma bin Laden was spirited over the mountains into Pakistan, before seeking refuge in Iran, where he initially lived in a safe house, according to Mr Soufan. He and his mother were eventually arrested by the Iranian authorities and incarcerated in a military camp.

As Hamza bin Laden grew older, he sought no special treatment within the group as Osama bin Laden’s son.

“He does not want to be treated with favouritism because he is the son of ‘someone,’” according to a 2010 letter from an aide to the elder bin Laden. “I promised him to plan some safe training for him: firing arms and with various weapons.”

Timeline of the Isis caliphate Show all 19 1 /19 Timeline of the Isis caliphate Timeline of the Isis caliphate ISIS began as a group by the merging of extremist organisations ISI and al-Nusra in 2013. Following clashes, Syrian rebels captured the ISIS headquarters in Aleppo in January 2014 (pictured) AFP/Getty Timeline of the Isis caliphate Abu Bakr Al-Baghdadi declared the creation of a caliphate in Mosul on 27 June 2014 Timeline of the Isis caliphate Isis conquered the Kurdish towns of Sinjar and Zumar in August 2014, forcing thousands of civilians to flee their homes. Pictured are a group of Yazidi Kurds who have fled Rex Timeline of the Isis caliphate On September 2 2014 Isis released a video depicting the beheading of US journalist Steven Sotloff. On September 13 they released another video showing the execution of British aid worker David Haines Timeline of the Isis caliphate The US launched its first airstrikes against Isis in Syria on 23 September 2014. Here Lt Gen William C Mayville Jnr speaks about the bombing campaign in the wake of the first strikes Getty Timeline of the Isis caliphate Isis militants sit atop a hill planted with their flag in the Syrian town of Kobani on 6 October 2014. They had been advancing on Kobani since mid-September and by now was in control of the city’s entrance and exit points AFP/Getty Timeline of the Isis caliphate Residents of the border village of Alizar keep guard day and night as they wait in fear of mortar fire from Isis who have occupied the nearby city of Kobani Getty Timeline of the Isis caliphate Smoke rises following a US airstrike on Kobani, 28 October 2014 AFP/Getty Timeline of the Isis caliphate YPG fighters raise a flag as they reclaim Kobani on 26 January 2015 VOA Timeline of the Isis caliphate Isis seized the ancient Syrian city of Palmyra on 20 May 2015. This image show the city from above days after its capture by Isis Getty Timeline of the Isis caliphate Kurdish forces are stationed on a hill above the town of Sinjar as smoke rises following US airstrikes on 12 November 2015 AFP/Getty Timeline of the Isis caliphate Kurdish forces enter Sinjar after seizing it from Isis control on 13 November 2015 AFP/Getty Timeline of the Isis caliphate Iraqi government forces make the victory sign as they retake the city of Fallujah from ISIS on 26 June 2016 Getty Timeline of the Isis caliphate Iraqi forces battle with Isis for the city of Mosul on 30 June 2017 AFP/Getty Timeline of the Isis caliphate Members of the Iraqi federal police raise flags in Mosul on 8 July 2017. On the following day, Iraqi prime minister Haider Al Abadi declares victory over Isis in Mosul Getty Timeline of the Isis caliphate Members of Syrian Democratic Forces celebrate in Al-Naim square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign Getty Timeline of the Isis caliphate Female fighters of the Syrian Democratic Forces celebrate in Al-Naim Square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign AFP/Getty Timeline of the Isis caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria in January 2019 They were among the last civilians to be living in the ISIS caliphate, by this time reduced to just two small villages in Syria’s Deir ez-Zor Richard Hall/The Independent Timeline of the Isis caliphate Zikia Ibrahim, 28, with her two-year-old son and 8-month-old daughter, after fleeing the Isis caliphate, on Saturday 26 January 2019 Richard Hall/The Independent

Hamza bin Laden married a daughter of a senior al-Qaeda leader, Abdullah Ahmed Abdullah, a wedding that was recorded on video found in the Abbottabad compound.

Thomas Joscelyn, a senior fellow with the Foundation for Defense of Democracies who has been studying al-Qaeda for years, said that the younger bin Laden’s role in the organisation was opaque.

“We don’t actually know what his real role was within al-Qaeda,” he said. “We know al-Qaeda was marketing him as a voice for a younger generation. You could see that when they would put out these audio messages from him.”

Mr Joscelyn cited evidence in the older bin Laden’s personal files that Hamza bin Laden had received elite training but that Osama bin Laden preferred that his son not take on a military role.

The younger bin Laden’s own ambitions, based on his audio recordings and letters to his father, indicated a desire to take an active role in al-Qaida.

“My beloved father, I was separated from you when I was a small child, not yet 13, but I am older now, and have attained manhood,” he wrote in a 2009 letter.

“But what truly makes me sad,” he added, “is the mujahedeen legions have marched and I have not joined them.”

Donald Trump slams navy Seal who took down Osama Bin Laden

In 2017, the United States officially listed him as a global terrorist. In a letter published by al-Qaeda the same year, Hamza bin Laden said his 12-year-old son had been killed, according to the SITE Intelligence Group. The circumstances were unclear.

Three Iraqi intelligence officials said there was a failed attempt to kill Hamza bin Laden that year.

In February, the State Department announced a $1m (£824,000) reward for information about his whereabouts, and Saudi Arabia revoked his citizenship. According to the US officials, he was already dead by then.