A State Department spokesman confirmed at a news briefing that Hallel Yaffa Ariel of Kiryat Arba, a Jewish settlement near Hebron, was a U.S. citizen, several media outlets reported.

Muhammad Nasser Tarayrah, a 17-year-old Palestinian from a nearby village, stabbed Ariel multiple times while she was sleeping. Civilian guards shot and killed him.

The State Department issued a statement condemning the murder “in the strongest possible terms.”

“This brutal act of terrorism is simply unconscionable,” the statement said. “We extend our deepest condolences to her family. We also understand another individual who was responding to the attack was wounded by the attacker. We extend our hopes for a quick and full recovery.”

Tarayrah, who had posted on social media about wanting to die as a martyr, reportedly jumped over the fence to enter the settlement, activating an alarm. Guards were already heading to the area when he started stabbing the teen, according to reports.

Meeting after the attack, Israeli Prime Minister Benjamin Netanyahu and Defense Minister Avigdor Liberman decided to close off the village where the assailant lived and revoke work permits in Israel for members of his village. They also launched the process to demolish his family home.

“The horrifying murder of a young girl in her bed underscores the bloodlust and inhumanity of the incitement-driven terrorists that we are facing,” Netanyahu said in a statement. “The entire nation deeply identifies with the family’s pain and declares to the murderers: You will not break us.”

Netanyahu said he expected the Palestinian leadership to “clearly and unequivocally” condemn the attack and to take immediate action to stop the incitement.

Lars Faaborg-Andersen, the head of the European Union office in Israel, tweeted: “Strongly condemn terror murder of Hallel Yaffa Ariel in Kiryat Arba this morning. No excuse for terrorism – LFA”.

Sari Bashi, the Israel and Palestine country director for Human Rights Watch, in a statement sent to journalists called the murder a “ghastly crime. The fact that settlements are illegal under international law does not make their inhabitants, the children as well as their parents, subject to lethal attack.

“At the same time,” she said, “the killing provides no legal justification for the Israeli government to punish the alleged attacker’s family members. Destroying the home of the attacker’s relatives and canceling their work permits is an unlawful collective punishment, causing suffering for family members who committed no crime.”