Sunday's attacks came only days after Israel lifted the closure it imposed Feb. 12 on the West Bank and Gaza Strip in the wake of reports that Hamas was planning to avenge the Jan. 5 Gaza Strip killing of master bombmaker Yehiya Ayash, also known as "The Engineer."

The deadly bombings came after security sources had warned not only that militants would avenge Ayash's death, but also that they would mark the second anniversary of the slaying of 29 Palestinian worshippers at a Hebron mosque by Jewish settler Baruch Goldstein.

The attacks, which struck a bus in the heart of Jerusalem and a soldiers' hitchhiking station near Ashkelon, killed at least 27 people, including the attackers, and wounding at least 79.

Israeli officials, who neither confirmed nor denied Hamas allegations that the Jewish state was behind Ayash's death, began easing the closure Feb. 15 to allow Palestinians older than 30 to return to their jobs in Israel. The closure was lifted Friday of last week.

But in the aftermath of Sunday's bombings, Israeli officials said they were reimposing the closure — and that it would continue for a long period.

"I don't think it is our duty to be concerned about the jobs of Palestinians," Internal Security Minister Moshe Shahal told Israel Radio. "Our obligation is only to the security of our citizens."

In the aftermath of the bombings, the Israel Defense Force chief of staff, Lt. Gen. Amnon Shahak, dismissed suggestions that Israeli security forces should have been more alert after the lifting of the closure.

"If it wasn't the 50th day [since Ayash's death] it would have been the 60th," he told reporters. "The Hamas does not need excuses. It has declared that it will act against us."

Police Commissioner Assaf Hefetz said security and political measures were being taken to combat terrorism, adding that the lifting of the closure and Sunday's attacks were not necessarily linked.

"Just because there was an attack immediately after the lifting of the closure does not mean it was because it was lifted," he told Israel Radio.

Despite these official statements, many Israelis criticized the government for lifting the closure at a time when there were reports that an attack was imminent.

In the Jerusalem attack on a No. 18 bus, a suicide bomber with a 22-pound bomb filled with nails and ball bearings killed 24 people and wounded 50 others, 10 of them seriously.

Two Americans, Matthew Eisenfeld, 25, of West Hartford, Conn., a second-year rabbinical student at the Jewish Theological Seminary in New York, and his girlfriend, Sara Duker, 22, of Teaneck, N.J., who was spending the year in Israel, were among the victims.

The attack came at 6:48 a.m., during Israel's rush hour, as the bus waited at a traffic light at Jaffa and Sarei Yisrael streets, some 900 feet from the central bus station.

The blast destroyed the bus, leaving only a charred, twisted frame. It damaged another local bus, No. 36, which was waiting behind it, as well as seven cars near the blast.

The second attack occurred in Ashkelon, less than an hour later, at a hitchhiking stop for soldiers. The suicide bomber in that attack killed one person and wounded 29, including six who were in serious condition.

The attack was reportedly carried out by a terrorist dressed in an Israeli army uniform who mingled with soldiers waiting for rides.

Hamas claimed responsibility for the attacks in a leaflet distributed and signed by the "cell of the new students of Yehiya Ayash."

The attacks came after a six-month halt by Hamas, which last attacked an Israeli target Aug. 21.

At that time, a suicide bomber detonated an explosion on the No. 26 bus in Jerusalem's northern neighborhood of Ramat Eshkol, killing four people, including American Joan Davenny, 47, of Woodbridge, Conn., a San Francisco native.

Sunday's bombings generated questions about the Palestinian Authority's ability to control Hamas terrorists.

Days before the latest attacks, Palestinian officials had reportedly reached an agreement with Hamas, declaring that the fundamentalists would halt attacks on Israel in return for the Palestinian Authority's protection from Israeli retaliation.

Israeli intelligence officials briefing the Cabinet on Sunday repeated their assessment that the Palestinian Authority was not doing enough to strike at the infrastructure of Hamas and other fundamentalist groups.

Prime Minister Shimon Peres, briefing reporters after Sunday's attacks, said he had telephoned Palestinian Council President Yasser Arafat and demanded that the Palestinian Authority stop the militants from carrying out their deadly missions.

Peres said Arafat had pledged to do so, and by Wednesday 120 activists from Hamas and Islamic Jihad had been arrested. Arafat also ordered Palestinians to hand in unlicensed weapons, or face confiscation and punishment.

In an impassioned speech before the Knesset on Monday, Peres vowed to "take all appropriate means in order to strike at terrorists everywhere."

"We will not rest until all of the lunatics and violent persons are properly punished," he added. "We will defeat them, even if the struggle is long and demanding."

Peres, who has enjoyed a wide lead in the polls against Likud rival Benjamin Netanyahu, was jeered when he visited the site of the Jerusalem blast.

Some Israelis reportedly chanted, "With blood and fire, we will throw out Peres," a reference to Israel's national elections on May 29.

Other Israelis gathered at the bomb site reportedly made death threats against the prime minister.

In the wake of the bombings, Israeli opposition parties refrained from openly attacking the government — but they called for a rethinking of the peace process.

The attacks elicited condemnations from world leaders, including President Bill Clinton, Egyptian President Hosni Mubarak, Jordan's King Hussein and British Prime Minister John Major.

Pope John Paul II condemned the bombings in his Sunday address and extended his condolences to the victims' families.