JERUSALEM (AP) — The U.N. envoy to the Middle East expressed concern Monday that Israel has not yet apprehended Jewish extremists suspected in a July arson attack on a Palestinian family's home that claimed the lives of a toddler and his parents.

The remarks by Nickolay Mladenov came after the toddler's mother died of wounds suffered in the blaze, becoming the third family member to die from the attack. Rihan Dawabsheh died in an Israeli hospital early Monday, a day after her 27th birthday.

On July 31, assailants hurled firebombs into a bedroom of the Dawabsheh family's home in the West Bank village of Duma in a pre-dawn attack, and sprayed graffiti of a Jewish star of David and the word "revenge" on the walls. Rihan's 18-month-old son Ali was burned to death in the attack, and her husband Saad died last month after being treated for burn wounds in an Israeli hospital.

The last remaining family member is the couple's 4-year-old son Ahmad, who is still undergoing treatment for severe burns at an Israeli hospital. A relative of the family, Amjad Dawabsheh, told Israeli Army Radio on Monday that relatives have not told the boy what happened to the rest of his family.

"How can we tell him, 'Your father and mother and brother died?'" he said.

The attack drew widespread condemnation and Israel pledged to get tougher on Jewish extremists in the West Bank suspected in attacks on Palestinians and their property.

Israel's Cabinet approved harsh measures to fight what Israeli leaders have called "Jewish terrorism," and three young settler activists were jailed for six months without charge, a measure used regularly against Palestinian detainees but rarely on Israelis.

Still, Israeli authorities have not announced arrests or identified suspects in the July arson attack. Israel has imposed a gag order on publishing details of the investigation into the arson attack.

Mladenov called for "justice" in a statement released after reports of Rihan's death.

"Acknowledging the wide condemnations issued at the time of the incident by Israeli and Palestinian leaders, I am nevertheless concerned by the lack of progress in identifying and prosecuting the perpetrators of this outrage," Mladenov said.

Israeli human rights groups say few police investigations into alleged Israeli settler crimes against Palestinians in recent years have resulted in indictments.

On Monday, Israeli police announced that two Israeli settler activists, an 18-year-old and a minor, were indicted on suspicion of setting fire to a Bedouin Arab tent in the West Bank last month to protest Israeli actions to round up Jewish extremists. No one was injured in the attack.

Police said the 18-year-old, Avi Gafni, had been living in a hilltop outpost in the West Bank and had been banned from the area three times in the last two years due to suspicions that he had been involved in arson attacks on Palestinian holy sites and property in the West Bank and Jerusalem.