Ahmed Jaabari, who was killed in an Israeli air strike on a car in Gaza City on Wednesday, was one of the most elusive and highest-ranking members of Hamas's armed wing.
Israel's Shin Bet domestic intelligence agency confirmed it targeted him in a joint strike with the army, saying he had been "directly responsible for executing terror attacks."
Hamas also announced the death, which sparked furious protests in Gaza City, with hundreds of members of the Islamic movement and its armed wing -- the Ezzedine al-Qassam Brigades -- chanting for revenge.
Jaabari deliberately kept a low profile, was rarely photographed and avoided being interviewed.
But a deal to secure the release of Israeli soldier Gilad Shalit brought him out of the shadows last year.
He allowed himself to be caught on camera on October 18, 2011 as he delivered Shalit to Egypt as part of a key prisoner exchange deal with the Jewish state.
The footage was broadcast instantly around the world, and showed Jaabari in civilian clothing, glasses in his shirt pocket, as he walked his Israeli charge to a car.
Jaabari hailed from a respected activist family in the Shejaiya neighbourhood of Gaza City, with close ally Abu Hudaifa describing him as confident in his own decisions and committed to following up personally on issues.
A history graduate from Gaza's Islamic University, Jaabari was arrested by Israel in 1982. At the time, he was an activist with Fatah, the secular Palestinian national movement which was to become a bitter rival of Hamas.
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It was in prison, where he spent 13 years for planning deadly attacks, where he met some of Hamas's top leaders, including Abdel Aziz al-Rantisi, Ismail Abu Shanab, Nizar Rayyan and Salah Shehadeh, and decided to join the movement.
Shehadeh led the Ezzedine al-Qassam Brigades until he was killed in a massive Israeli air strike in July 2002, after which he was replaced by Mohammed Deif.
Several months later, Deif was badly wounded in another Israeli strike and went underground, leaving Jaabari as the operational head of the armed movement at the height of the second intifada, or Palestinian uprising.
Known in Gaza as "the general" or the "chief of staff," Jaabari could occasionally be spotted walking alone in the street.
But, as one of the names highest on Israel's most wanted list, Jaabari was known for taking almost obsessive care when it came to his personal security.
Jaabari had previously been the apparent target of more than one Israeli assassination attempt, including a 2004 air strike that killed his eldest son Mohammed, along with his brother and several of his cousins.
He was also targeted by the Palestinian security forces, who arrested him in 1998 and held him for nearly two years because of his activities with Shehadeh and Deif.
After Jaabari took over the day-to-day running of operations, the armed group became increasingly professional.
He was also credited with playing a leading role in the Islamist movement's forcible takeover of Gaza in summer 2007, which saw its militants expelling Fatah forces after a week of bloody fighting.
In addition to his leadership role in Ezzedine al-Qassam, Jaabari was a member of the movement's political leadership and the founder of Nur, an association to help "martyrs and prisoners."
He had two wives, including a daughter of his mentor, Shehadeh.