Hamas accused Israeli agents of assassinating one of the Palestinian militant group's veteran operatives, and vowed to retaliate.
The militant group identified its slain figure as Mahmoud al-Mabhouh, one of the founders of Hamas' military wing. Al-Mabhouh, 50, was responsible for hundreds of deadly attacks and suicide bombings targeting Israelis since the 1980s.
Hamas blamed Israel for his death, but gave no details on how al-Mabhouh was killed.
The Israeli newspaper Ha'aretz, however, quoting al-Mabhouh's brother, reported that a medical team that examined Mabhouh had determined that he died after a massive electric shock to the head, and that there was evidence of strangulation.
Israeli officials had no immediate comment on the claims.
Israeili newspaper Ha'aretz and the Associated Press contributed to this article.