The brutal killings of four worshippers - including three Americans - in a Jerusalem synagogue by two Palestinians wielding meat cleavers and a gun appears to be a lone wolf attack, Israel's national police commissioner has said.
The violence was met with celebration from Palestinians, as revelers in the Gazan city of Rafah handed out candy and brandished axes and posters of the suspects in praise of the deadly attack, according to the Jerusalem Post.
National Police Commissioner Yochanan Danino told the newspaper that a preliminary investigation of the attack indicates a grassroots, independent effort
Hamas said the attack was motivated by revenge after a Palestinian bus driver was found hanged in his vehicle earlier in the week, and called for more violence.
But Israeli police believe Youssef al-Ramouni, 32, who was found dead at the start of his bus route on Sunday, committed suicide, according to the Jerusalem Post.
Three of yesterday's victims held dual US-Israeli citizenship and the fourth man was a British-Israeli national.
Israeli prime minister Benjamin Netanyahu pledged to respond with a "heavy hand" and accused Western-backed Palestinian president Mahmoud Abbas of inciting violence in Jerusalem.
Abbas condemned the attack, which took place after weeks of unrest fuelled in part by a dispute over Jerusalem's holiest shrine.
A worshipper at the service in the Kehillat Bnei Torah synagogue in an ultra-Orthodox neighbourhood of Jewish West Jerusalem said about 25 people were praying when shooting broke out.
"I looked up and saw someone shooting people at point-blank range. Then someone came in with what looked like a butcher's knife and he went wild," Yosef Posternak said.
Photos distributed by Israeli authorities showed a man in a prayer shawl lying dead, a bloodied butcher's cleaver on the floor and prayer books covered in blood.
US president Barack Obama said in a statement: "I strongly condemn today's terrorist attack on worshippers at a synagogue in Jerusalem, which killed four innocent people, including US citizens Aryeh Kupinsky, Cary William Levine and Mosheh Twersky, and injured several more.
"At this difficult time I think it's important for both Palestinians and Israelis to try to work together to lower tensions."
Israeli police spokesman Micky Rosenfeld said the two assailants, both from Israeli-annexed East Jerusalem, were shot dead by police in a gun battle outside the synagogue. Netanyahu said Israel would demolish their homes.
Israel's ambulance service said at least eight people were seriously wounded.
The Popular Front for the Liberation of Palestine militant group said it carried out the attack, which it called a "heroic operation".
The four dead - Twersky (59), Kupinsky (43), Levine (55) and Avraham Shmuel Goldberg, a 68-year-old British-Israeli, were all ordained rabbis.
Palestinian radio described the attackers as "martyrs". Palestinian media named the assailants as Ghassan and Udai Abu Jamal, cousins from the Jerusalem district of Jabal Mukaber, where clashes broke out as Israeli security forces moved in to make arrests.