An Israeli police officer holds his weapon near the scene of an earlier attack on a Jerusalem synagogue. Reuters /Ronen Zvulun

Israeli Prime Minister Benjamin Netanyahu vowed to "respond harshly" to Tuesday’s attack in Jerusalem -- the deadliest in six years. Israel's Public Security minister said he would ease gun restrictions for “self-defense,” according to the English-language edition of Al-Akhbar. Meanwhile, Israeli forces have already detained 12 family members of the two men accused of attacking the synagogue, according to Palestinian Ma’an news agency.

Cousins Ghassan Abu Jamal and Oday Abu Jamal are accused of committing what U.S. Secretary of State John Kerry called an act of “pure terror.” The two men took meat cleavers and pistol into an East Jerusalem synagogue and attacked worshippers during morning prayer. Four people were killed and several others wounded before law enforcement officers fatally shot them. There were about 25 people in the synagogue, the Associated Press reported.

Family members said in published reports that police forces searched the men’s homes in the Jabal al-Mukabbir city of East Jerusalem after the attack and then detained Ghassan Abu Jamal's wife, mother, and four brothers. Oday Abu Jamal's brother was also detained.

"Two terrorists, apparently from East Jerusalem, entered a yeshiva (Jewish seminary) in Har Nof and attacked worshipers with axes and a pistol," Israeli police spokeswoman Luba Samri said. "There are four dead and six injured, among them two Israeli policemen.”

Israeli Public Security Minister Yitzhak Aharonovitch announced on public radio that he would lighten gun restrictions “in the coming hours” for Israeli citizens and law enforcement officials who already have a gun license, citing a higher need for self-defense, according Al-Akhbar.

Israeli law enforcement is thought to have more than 26,000 firearms, according to Gun Policy statistics from 2012. The same statistics show that there are an estimated 500,000 guns in civilian possession.

The cousins were thought to be members of the Popular Front for the Liberation of Palestinian, a militant group based in the area. The group confirmed that the men were members but didn't claim responsibility for the attacks, AP reported.