A “cruel” fanatic who fooled Israel by lying low before launching the most devastating terrorist attack in its history, Yahya Sinwar is now perhaps the world’s most-wanted terrorist.

As Hamas’s highest-ranking official in Gaza, he is considered so important to the terrorist group’s operations and war effort that Israel this week promised a bounty of $400,000 (£315,212) for any information leading to his capture.

As the Israeli ground operation in Gaza following the Oct 7 attacks masterminded by Sinwar approaches its third month, catching him has become one of Israel’s main war aims.

Believed to be hiding in the labyrinth of tunnels beneath Gaza, he has so far eluded capture.

Patience is wearing thin. Earlier this week, Israeli cabinet ministers berated the Israel Defense Forces (IDF) for failing to catch Sinwar in its Gaza assault.

But who is Sinwar? How did he take power in Gaza? And why did he devise and launch the brutal attacks on Israel that left more than 1,200 and led to some 240 Israelis being taken hostage, about half of whom remain in Hamas captivity.

At 61, with a slight build and diminutive stature, Sinwar stands out among the Hamas leadership, many of whom have long since left behind the hardships of Gaza in favour of luxurious lives in exile in Qatar.

While other leaders condemned the devastating Israeli air strikes that rained down on Gaza during the 2021 war from the comfort of their five-star hotels, Sinwar made a point of staying put, famously posing on a sofa in his bombed-out house in an act of defiance at the end of the hostilities.

Sinwar’s personal story echoes what many Palestinian families went through around the time of the creation of the state of Israel in 1948, when an estimated 700,000 people were displaced from their homes.

The second most powerful member of Hamas after Ismail Haniyeh, the group’s overall leader, Sinwar was born in a refugee camp in Khan Younis in Gaza after his father’s family was expelled from the city of Majdal, which later came to be known as Ashkelon.

In recent decades, the city of Sinwar’s ancestors has become a recurrent target for Hamas rockets.

After an impoverished childhood in which his family had to rely on UN aid, Sinwar attended the Islamic University of Gaza for Arabic studies and soon dedicated himself to militancy.

He had barely turned 26 when he was detained after being injured when an improvised explosive device he was making accidentally went off.

It was only when he was imprisoned in Ashkelon that Israeli investigators discovered his involvement in the killings of four fellow Palestinians accused of collaborating with Israel, after which he became known as the “Butcher of Khan Younis”, a nickname that persists to this day.

He had, it turned out, co-founded an organisation within Hamas for the special purpose of tracking down collaborators.

Israeli newspapers ran excerpts from his testimony after he was sentenced to four life sentences in 1989.

He would describe convincing a Hamas member to expose his own brother as a collaborator before burying him alive.

Sinwar would spend the next 22 years in jail. There, he befriended fellow Palestinian prisoners and built a small group of confidants who would smuggle cell phones and interrogate new inmates to make sure they were not informants.

Several Hamas leaders have said that securing the release of those Palestinians who remain in Israeli jails was one of the main aims of the Oct 7 attacks.

So far, some 240 Palestinians have been released by Israel in exchange for 105 Israelis.

Israelis and fellow prisoners also described Sinwar’s charm offensive directed at his jailors, as he demonstrated a keen interest in Israeli culture and politics, and learnt to speak fluent Hebrew – the roots of a deception that continued to take shape until Oct 7.

But Sinwar has long had a reputation for cruelty. One of his former dentists recently revealed how he had recruited countless Palestinians to his cause while languishing in prison.

Speaking to Germany’s Bild tabloid last month, Dr Yuval Biton called his former patient a charismatic but menacing figure.

“I know how cruel he is, I have never underestimated his abilities, but unfortunately others have ... he knows us very well, he follows Israeli society, our politics, our debates,” said Dr Biton, who treated Mr Sinwar in jail in the 1990s.

In one significant incident, Mr Sinwar was asked shortly before his release to sign papers renouncing terrorism.

“He asked what it was ... then he refused to sign the paper, and all subsequent Hamas prisoners then refused as well,” Dr Biton recalled.

Israeli media reports also claimed that during his imprisonment Sinwar received surgery for an illness – a brain tumour, according to one former official – that saved his life.

Sinwar was eventually released in 2011 as part of a landmark prisoner exchange that freed Gilad Shalit, an Israeli soldier held hostage by Hamas.

After returning to Gaza, Sinwar quickly rose through the ranks of Hamas with the help of his brother Mohammed.

In 2017, he was elected head of the terrorist organisation in Gaza and became the enclave’s de-facto ruler in 2017 shortly before Haniyeh, the head of Hamas’s political bureau, left for Qatar.

In recent years, Sinwar sought to patch up relations with neighbouring Egypt and signalled that he may even look for reconciliation with Israel – though this turned out to be a ruse.

In a rare interview with an Israeli newspaper in 2018 he claimed: “I don’t want any more war.”

But he kept building a circle of allies and pumped money into Hamas’s military wing that orchestrated the Oct 7 attacks.

Earlier this month, Israel’s Shin Bet security agency published footage from the interrogation of Yousef Al Mansi, a former Hamas communications minister, who said that Sinwar “feels like he is above everyone else”.

He added: “He makes decisions without consulting anyone.”

The IDF leaflets offering a bounty for Hamas leaders also included rewards for information on Sinwar’s brother Mohammed, the head of Hamas’s military wing and the commander of the Khan Younis Brigade.

Israel has its sights on other top Hamas leaders, such as Mohammed Deif, but it is Sinwar, with his story of lengthy incarceration and sudden release, that has captured the imagination of Israelis.

“Clearly Sinwar has always been a bit of a bogeyman for Israel because he was released in the prison exchange in 2011… there has always been a degree of obsession with him,” said Hugh Lovatt, a senior Middle East analyst at the European Council on Foreign Relations.

“Part of that may also be that the Israeli anger needs to be channelled and personified in the same way that Osama bin Laden was the object of the US anger.”

Mr Lovatt is sceptical of claims that catching Mr Sinwar would paralyse Hamas.

“The killing of Mohammed Deif or Yahya Sinwar will not lead to the collapse of [Hamas]. It would further coalesce support around the movement in the West Bank and elsewhere.”

In the years preceding the Oct 7 attacks, the Israeli government came to consider Hamas as a lesser threat than the Palestinian Authority in the West Bank because they appeared to have trusted Sinwar’s assurances that he did not want war.

Yet one recent speech appeared to directly allude to the disaster that was to come to Israel.

Last summer, Sinwar told Palestinians at a rally in Gaza City to “rise up as a gale to defend Al-Aqsa”, the landmark mosque in Jerusalem, warning Israel that Palestinians will “come in a roaring flood, in rockets without end and in a flood of soldiers limitless in numbers”.

When the first reports about the surprise attacks on Israel started to pour in on Oct 7, Hamas said that it was a special operation called Al-Aqsa Flood.