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JERUSALEM -- Militants in the Gaza Strip unleashed dozens of rockets on southern Israel late Monday, setting off air raid sirens and forcing hundreds of thousands of Israelis to stay indoors as the military rushed more forces to the border and warned that even heavier fighting looked likely.
The latest violence came as Israel pressed forward with its investigation of six Jewish youths suspected of abducting and killing a Palestinian teenager, and Israeli leaders sought to calm an emotional debate over whether the country's politically charged atmosphere led to the gruesome crime. An Israeli official said three of the youths had confessed to the attack.
Tensions have been high since three Israeli teenagers kidnapped June 12 in the West Bank were later found dead, followed by last week's slaying of the Palestinian youth in what many suspect was a revenge attack. Throughout the unrest, Gaza militants have launched more than 200 rockets and mortars into Israel, including close to 100 on Monday alone.
Israel has responded with dozens of airstrikes, but has not been able to halt the attacks. Eight Palestinian militants were killed in fighting Monday, the highest death toll yet.
Dozens of rockets were fired Monday, including 40 launched in a single hour after nightfall, setting off air raid sirens up to 50 miles from Gaza, the military said.
Twelve rockets were intercepted by rocket-defense batteries, it added, while the others landed in open areas. It was the deepest penetration of rocket strikes in the current round of fighting and raised the likelihood of an even tougher Israeli response.
Maj. Gen. Yoav Mordechai, a senior military official, gave a special interview in Arabic to Al-Jazeera, warning that Hamas would bear the consequences for the escalation.
Among the dead were six Hamas militants who Israel said were killed in an accidental blast in a tunnel packed with explosives. Hamas, the Islamic militant group that controls Gaza, vowed revenge, saying "the enemy will pay a tremendous price."
Lt. Col. Peter Lerner, an Israel Defense Forces spokesman, said the army was moving two infantry forces to the Gaza border and had received authorization to mobilize up to 1,500 reservists.
The deaths of the Hamas militants had made a "substantial influence" on the situation, he said.
"There is a potential of deterioration due to their death. Therefore the IDF has to continue to reinforce capabilities in the south, with the potential that things could escalate further," he said.
Israeli security officials said Israel was leaning against a massive operation and would likely increase the pressure gradually with stronger and more numerous retaliatory attacks. They spoke on condition of anonymity because they were not authorized to discuss internal military deliberations with reporters. Israeli Cabinet ministers voted in favor of more intense airstrikes, Channel 2 TV said.
In Washington, State Department spokeswoman Jen Psaki said the U.S. condemns the rocket fire. "We also support Israel's right to defend itself against these attacks," she said.
During the day, lines of Israeli tanks and buses were gathered near the border area as soldiers milled about. Late Monday, with roads in southern Israel all but empty, a flatbed truck carrying an armored vehicle made its way toward the border area.
Israel has launched two broad military operations in Gaza in the past five years, most recently in 2012, when eight days of heavy fighting ended in an Egyptian-mediated truce.
The increased rocket fire followed the killing of Mohammed Abu Khdeir, a 16-year-old Palestinian boy from east Jerusalem who was abducted and burned to death last week.
Israeli officials Sunday announced the arrests of six Jewish youths in the killings -- ruling out earlier theories that criminal activity or personal reasons might have been a factor. They said the suspects, including some minors, were from the Jerusalem area.
Abu Khdeir's death triggered several days of violent protests in Arab areas of Jerusalem and northern Israel as Palestinians accused Israeli extremists of killing the boy to avenge the earlier deaths of the Israeli teens.
Israel's Shin Bet agency, which handles top security cases, imposed a gag order, blocking key details, including the identities and backgrounds of the youths, as well as most of the evidence from being made public.
An Israeli official said three of the youths had confessed to the crime and even re-enacted it for authorities. The official spoke on condition of anonymity due to the gag order.
The official said the suspects included two brothers, and that one of the suspects is the son of a rabbi. Three minor suspects were allowed to meet with lawyers Monday, while the primary suspects were not.
Naftali Werzberger, a lawyer for one of the suspects, said he didn't even know what his client looked like, adding that the young man was prohibited from receiving details that he is being represented by a lawyer.
"The families are in shock," Werzberger added. "No one among the suspects has a violent past or tendency and they can't explain how we got to this point where their children were arrested as suspects for acts that are very, very serious."
Despite the gag order, some additional details have begun to emerge.
Israel's Channel 10 TV said the suspects were the son and five grandsons of a prominent rabbi in Jerusalem, without further identifying him. It said two suspects used the rabbi's car for the kidnapping without his knowledge. It added that a Jerusalem district court presented its suspicions Monday against the suspects, including "belonging to a terror organization." It did not elaborate, and Israeli officials did not confirm the details in the report.
The news that the suspects were Jewish set off nationwide soul-searching over how Israelis could possibly carry out such a gruesome crime. A preliminary autopsy found that Abu Khdeir was still alive when he was set on fire.
"I am ashamed on behalf of my nation and grieve with you," President Shimon Peres told Abu Khdeir's father, Hussein, in a phone call. "The only thing left for all of us to do is to ensure that no more children are murdered, and no more tears are shed by mothers."
Peres, a Nobel peace laureate, was among top Israeli officials, including Prime Minister Benjamin Netanyahu, who reached out to the family in person or by telephone.
Netanyahu, who has condemned Abu Khdeir's death and tried to calm the public, said he had expressed his condolences to the family.
"I would like to express my outrage and that of the citizens of Israel over the reprehensible murder of your son," a statement quoted Netanyahu as saying.
Rachelle Fraenkel, the mother of one of the slain Israeli teenagers, said that even from the "abyss" of her own pain, she could not describe her distress over the killing of the Arab boy.
"No mother and father should endure what we are going through now. We feel the pain of Mohammed Abu Khdeir's parents," she said.
Also Monday, Israeli police said a border policeman had been temporarily suspended from special operations and transferred to a different position until the completion of an investigation into the apparent police beating of Tariq Abu Khdeir, a Palestinian-American teenage cousin of the murdered Palestinian youth.
Despite his home detention, Tariq Abu Khdeir traveled to the West Bank town of Ramallah to meet President Mahmoud Abbas.
Daniel Estrin contributed to this report.