Said Siyam is the most senior leader of the Islamist militant organisation Hamas to have been killed since the beginning of the Israeli offensive on the Gaza Strip.
Analysts say Siyam's death will be a blow to Hamas security forces in Gaza
Hamas television said Mr Siyam, his son, brother Iyad and his brother's family, had died when an Israeli missile struck his brother's home in Gaza City on 15 January.
The Israeli military say he was killed in a joint operation carried out by the Israeli Southern Command and the Shin Beth internal security agency.
The 49-year-old was interior minister in the Hamas government and oversaw thousands of members of the Executive Force, a paramilitary organisation which played a key role in the group's violent take-over of the coastal territory in June 2007.
He is also said to have been one of three members of the "collective leadership" of Hamas in Gaza since 2004, which was formed after Sheikh Ahmad Yassin and Abdel-Aziz Rantissi were killed by Israel.
Mr Siyam was born in the Shati refugee camp near Gaza City in 1959 after his parents fled their home in the village of al-Jura, near what is now the Israeli town of Ashkelon during the 1948 Arab-Israeli war.
After completing his secondary education in Gaza City, Siyam gained a diploma in sciences and maths, followed by a bachelor's degree in Islamic education from the Al-Quds Open University.
Born: 1959, Shati refugee camp
Six children - two boys, four girls
Member of Hamas Political Bureau
Elected to Palestinian Legislative Council in 2006; top Hamas MP
Became Hamas interior minister in 2007, after serving one year as Palestinian interior minster
Said to have become member of Hamas "collective leadership" in Gaza after 2004 killings of Sheikh Yassin and Abdel-Aziz Rantissi
Created Hamas Executive Force, a paramilitary force which played key role in take-over of Gaza in June 2007
Killed in Israeli air strike on 15 January 2009
He then became a teacher at local UN-run schools, a profession in which he remained until 2003, despite his prominence within the Palestinian Islamic Resistance Group, Hamas.
It is not known when Mr Siyam joined Hamas, which was formed by Palestinian Muslim Brotherhood leaders following the outbreak of the first mass uprising - or intifada - against the Israeli occupation in 1987.
However, Palestinian reports say he was arrested at least four times by the Israeli authorities between 1989 and 1992 for participating in Hamas activities.
And later in 1992, Israel deported him along with senior Hamas leaders Abdel-Aziz Rantissi, Mahmoud Zahhar and Ismail Haniya, and more than 400 other activists, to South Lebanon.
Mr Siyam was again detained for a few months in 1995, not long after his return to Gaza, however this time by the Palestinian Authority, which was dominated by Hamas's main rival, Fatah.
In subsequent years, Mr Siyam rose through Hamas's ranks, preaching at a local mosque, and reportedly becoming close to the group's spiritual leader, Sheikh Yassin, who was released from an Israeli prison in 1997, and its exiled political leader, Khaled Meshaal.
The Executive Force was set up to rival the Palestinian security services
After ending his teaching career in 2003, he became a member of the group's Political Bureau and head of its foreign affairs department, leading negotiations with Egyptian and Iranian officials, with whom he enjoyed warm relations.
In March 2004, Sheikh Yassin and his successor, Abdel-Aziz Rantissi, were assassinated within days of each other. Wary of further attacks, Hamas kept the appointment of Rantissi's successor secret.
But Palestinian sources said Mr Siyam had been appointed to the group's "collective leadership" with Mahmoud Zahhar and Ismail Haniya.
Two years later, he won election to the Palestinian Legislative Council, collecting the most votes for any single candidate.
Hamas won a total of 76 seats out of 132 in the parliamentary poll, challenging Fatah's long-held dominance, and was asked to form a new government.
Mr Siyam was appointed interior minister, a post which put him in control of three of the Palestinian Authority's five security services, and made him responsible for contacts with Israel's security services.
Mr Siyam died when an Israeli missile struck his brother's home in Gaza City
Hamas's refusal to recognise Israel and renounce violence meant, however, that the cabinet's ability to govern was heavily restricted by President Mahmoud Abbas, the Israeli government and the international community.
However, Mr Siyam used the opportunity to form the Executive Force, a force to rival the Fatah-dominated Palestinian security forces in Gaza.
He was replaced as interior minister in March 2007 after a national unity government was formed to end the deadlock, but he remained in control of the Executive Force and used it to drive Fatah from the territory three months later.
The Executive Force, which has 13,000 members, was praised by many Gazans for helping to bring order to the territory, but others have accused it of practising torture and illegally detaining rivals.
Analysts say Mr Siyam's death, though a blow to its security apparatus, is unlikely to end the Islamist group's defiance in the face of the Israeli offensive.
"Some of our leaders will fall, some of our people will fall, but the flag of resistance won't fall," Hamas spokesman Sami Abu Zuhri told the Associated Press after Mr Siyam's death.