Clashes broke out between Israeli Police and Palestinian citizens in East Jerusalem Tuesday, just hours after two Palestinian extremists killed four people in a synagogue in an ultra-orthodox Jerusalem neighborhood, Israeli authorities said. A Palestinian man was stabbed in the leg in North Jerusalem as the situation grew increasingly volatile in Jerusalem, East Jerusalem and other Israeli and Palestininian areas, police told Al Jazeera. The network also reported Israeli Prime Minister Benjamin Netanyahu had called for the demolition of the attackers' homes.

The FBI will launch an investigation into the attack, as three of the four people killed were dual Israeli-U.S. citizens, an American law enforcement official told CNN. Six people were also injured in the synagogue attack. The U.S. Consulate in Jerusalem said the three Americans killed were Aryeh Kupinsky, Cary William Levine and Mosheh Twersky, the Associated Press reported. The fourth victim was a dual Israeli-British citizen identified by Israeli authorities as Avraham Goldberg. Thousands of people mourned their deaths in a quickly planned joint funeral for Kupinsky, Levine and Goldberg near the scene of the attack Tuesday, the AP reported.

Netanyahu lashed out, promising to “respond harshly” to the attack, allegedly committed by Palestinian cousins Ghassan Abu Jamal and Oday Abu Jamal. Victims were attacked inside the synagogue with knives and axes.

The East Jerusalem men, who were members of the sporadically violent Popular Front for the Liberation of Palestine (PFLP) group, were shot to death by Israeli police officers who responded to the scene. The PFLP has been a consistent impediment to peace in Israel since the 1960s, claiming responsibility for attacks ranging from hijackings to assassinations and suicide bombings.

Outrage and sadness over the attack spread swiftly around the world Tuesday, with leaders from New York to London decrying the attack and calling for justice.

“Brooklyn mourns the loss of life resulting from today’s terror attack in Jerusalem, and I personally condemn those individuals responsible for this most inhuman brutality,” Eric L. Adams, borough president of Brooklyn, New York, which is home to one of the largest Orthodox Jewish communities outside of Israel, said in a statement. “Violence anywhere is unacceptable everywhere. The international community must stand united in demanding the security of innocent civilians.”

Palestinians and Israelis from Jerusalem to the West Bank took to the streets to protest the attack erupting from the outpouring of emotion and frustration. Police fired tear gas at rock-throwing residents as they made a number of arrests after the attack Tuesday in a largely Arab neighborhood of Jerusalem, Al Jazeera reported.

The synagogue attack is the worst in Jerusalem since March 2008 when a man gunned down eight seminary students, according to CNN.