A grisly trail of toys and blood led paramedics to the first three bodies: a mother, father and their 4-month-old infant, stabbed to death in their bed. In the next room, medics say they found the body of an 11-year-old sibling.

Finally, with growing dread, they reached the last bedroom, where a 4-year-old boy with knife wounds and a faint pulse was fighting for his life, ambulance workers said Saturday on Israel Radio. The medics worked frantically, but unsuccessfully, to resuscitate the toddler. One called it a scene of "incomprehensible horror."

The brutal killings Friday night of five Jewish settlers in the tightly guarded compound of Itamar, southeast of the West Bank city of Nablus, sent shockwaves through Israel, sparking worries of renewed violence in the Palestinian territories and heightening fears of retaliation from the Israel Defense Forces or angry settlers.

Israeli authorities suspect that the killings, the deadliest attack inside a settlement in several years, were either a strike by Palestinian militants or a revenge attack by residents of the West Bank village of Awarta, where two Palestinian teenagers were shot to death a year ago as they collected garbage near Itamar.

Israeli leaders condemned the attack on Udi and Ruth Fogel and three of their children and launched a massive manhunt throughout the region, setting up checkpoints, sealing off Palestinian villages, imposing curfews and conducting house-to-house searches, Palestinians said.

"Israel will not stand by idly after such a despicable murder and will act vigorously to safeguard the lives of the citizens of Israel and punish the murderers," Prime Minister Benjamin Netanyahu said Saturday.

The White House offered its condolences to the victims' family, saying in a statement, "There is no possible justification for the killing of parents and children in their home."

Palestinian Authority Prime Minister Salam Fayyad also criticized the killings. "As we have always rejected violence against our people, we reject it against others and we condemn it."

Some Palestinians in Awarta, who have clashed repeatedly with Itamar settlers over the years, agreed that the killing of children was reprehensible.

"If it had been an attack against a soldiers' outpost, it might have been understandable, but a person who would kill a baby is not a human being," said Awarta shopkeeper Zahid Quwark, 53.

In the past, Palestinian militants have defended such attacks against Jewish settlers in the West Bank, describing the settlers as combatants in the conflict rather than civilians.

Israeli soldiers appeared to be focusing their efforts on family members of the two Palestinians killed last March. At the time, Palestinians had complained that the unarmed youths were killed by settlers from Itamar, although Israeli soldiers said they shot the teens. A military investigation was opened into the incident.

Several male relatives in the family were arrested Saturday and their home remained surrounded by Israeli soldiers.

"We will not rest until we capture the murderers," Lt. Gen. Benny Gantz, Israel's new army chief of staff, said after visiting the family's home in Itamar.

The hilltop settlement, home to 1,000 ideological and religious families, remained closed by the military Saturday as investigators worked to understand how infiltrators managed to evade its electric fence and intruder-detection devices.

The attacker, or attackers, appears to have climbed over or cut through the fence Friday night and crawled through a window of the family's home, according to Israeli media reports.

Two other children were also in the home, but escaped harm by hiding. Around midnight, another daughter arrived home after a youth outing and discovered the scene with a neighbor.

Tensions between settlers and Palestinian villagers have been escalating for weeks. Founded in 1984, the Itamar settlement sits on land that was once controlled by the village of Awarta, said Awarta Mayor Qais Awwad.

In recent weeks, Palestinians have accused settlers in the area of chopping down hundreds of olive trees, burning Palestinians' cars and shooting at villagers. Last week, Israeli soldiers were accused of using live gunfire to quell one clash, injuring 10 Palestinians and one settler.

Itamar's settlers are considered among the most fervent, believing Israel has a historic and religious right to absorb the West Bank, which Israel seized during the 1967 Middle East War.

Most of the international community, however, views Israel's settlements as illegal and has called for Israel to end the occupation by allowing Palestinians to build their own state on the land.

Over the last two years, Israel has lifted many roadblocks and checkpoints in the area surrounding Nablus, including one at Hawara, near the highway turnoff to Itamar and Awarta. Now settlers are demanding the military checkpoints be reinstalled.

"This was a barbaric act that shows the true face of the so-called Palestinian struggle for liberation," said Danny Dayan, head of the Yesha Council, a settler group.

He said Israel should suspend its cooperation with Palestinian security forces in the West Bank and do more to protect settlers living here.

The last deadly attack on a West Bank settlement was in August, when four settlers were killed near Hebron. A group affiliated with Hamas, the Islamist group that controls the Gaza Strip, claimed responsibility for that attack.