A remote-controlled bomb tore apart an armored vehicle in a U.S. diplomatic convoy Wednesday, killing three American security guards and wounding a fourth in the first deadly attack on a U.S. target in the Palestinian territories.

The attack, on a convoy of U.S. Embassy diplomats entering Gaza to interview Palestinian candidates for a Fulbright scholarship (search), was a dramatic departure from typical militant operations, which usually target Israeli soldiers and civilians. It was almost certain to lead to greater U.S. pressure for a Palestinian crackdown on militant groups.

"Palestinian authorities should have acted long ago to fight terror in all its forms," President Bush said, blaming Palestinian officials for the attack.

Secretary of State Colin Powell told Palestinian Prime Minister Ahmed Qureia (search) when Qureia telephoned with condolences that the Bush administration could not move forward on establishing a Palestinian state unless violence and terrorism were eliminated.

Qureia promised to track down those responsible, and Palestinian leader Yasser Arafat (search) condemned the attack as an "awful crime."

There was no claim of responsibility, and the largest militant groups — Hamas, Islamic Jihad and the Al Aqsa Martyrs Brigade (search) — quickly distanced themselves from the attack. Palestinian security sources said they were focusing on small groups who receive funding from abroad, including from Iran.

The attack came four months after a previous assault on a U.S. bulletproof vehicle in Gaza. That attack, which was not publicly revealed until Wednesday, did not cause any injuries, said U.S. Ambassador Dan Kurtzer, providing no other details.

Kurtzer said those killed Wednesday were U.S. citizens working on contracts to provide security for the U.S. Embassy in Tel Aviv.

The State Department identified the slain Americans as John Branchizio, 36; Mark T. Parson, 31; and John Martin Linde Jr., 30 — all employees of DynCorp (search), a Virginia-based security firm. The wounded American was initially treated at a Gaza hospital before being transferred to a hospital in the southern Israeli city of Beersheba.

After the bombing, the U.S. government advised its citizens to leave the Gaza Strip (search). Kurtzer said 200 to 400 Americans, some of Palestinian descent, work in the Gaza Strip, many for aid groups.

U.S. diplomats at meetings in the West Bank were immediately brought back to Jerusalem, and it remained unclear if U.S. travel in the West Bank and Gaza would be further curtailed.

U.S. investigators who went to the scene hours after the attack were chased away by Palestinians hurling stones.

The investigation will be a cooperative probe involving Israeli police and the FBI, according to FBI officials. FBI agents do not intend to go into Gaza immediately; instead, they will rely on the Israelis to collect and preserve evidence, with the FBI doing the detailed examinations of what is found.

The diplomats were riding in a three-vehicle convoy — escorted by Palestinian police — en route to Gaza City when the blast went off about 10:15 a.m.

An Associated Press reporter saw a gray wire with an on-off switch leading from the scene of the attack to a small concrete room at the side of the road. The blast took place about a mile south of the Erez crossing between Israel and Gaza.

The blast hit the third vehicle in the convoy just after it passed a gas station, said Mohammed Radwan, a Palestinian taxi driver who was at the station at the time.

"The first two cars drove quickly and stopped far from the explosion. Palestinian security people jumped out of the car and rushed to the car that had blown up. ... I saw two people covered with blood lying next to the car," he said.

The explosion gouged a deep crater into the unpaved road, nearly tore the vehicle in half and flipped it over. The pavement was stained with blood and littered with bits of flesh that were collected by Palestinian paramedics.

U.S. and Israeli officials said the attack underscored the need to dismantle Palestinian militant groups — a requirement of the stalled, U.S.-backed "road map" peace plan that Palestinian leaders have refused to carry out.

"The failure to create effective Palestinian security forces dedicated to fighting terror continues to cost lives," Bush said in a statement. "There must be an empowered prime minister who controls all Palestinian forces — reforms that continue to be blocked by Yasser Arafat."

At a news conference, Powell said he made clear to Qureia, the Palestinian prime minister, "that the only way forward is for him to get sufficient political authority to deal with this crisis in the Palestinian community, to gain control of all the security forces of the Palestinian Authority and to use those forces to go after terrorism."

Zalman Shoval, an adviser to Prime Minister Ariel Sharon (search), said: "What happened is evidence that no one is immune, unfortunately, to Palestinian terrorism, even when we are talking about the representatives of ... the United States, whose entire goal was and remains to advance a peace agreement between the sides."

U.S. convoys of armored black and silver Chevrolet Suburbans travel in Gaza almost daily and usually take the same route on the main north-south road in the strip. The convoys are easily identifiable: They are escorted by Palestinian police and have diplomatic plates. The color and make of the vehicles are unique to U.S. officials.

Although 49 Americans, many with dual citizenship, have been caught in the crossfire in the past three years of fighting, Palestinian militants have never killed Americans in a targeted attack.

Hamas (search), Islamic Jihad (search) and Al Aqsa (search) all issued statements denying responsibility for the attack, saying it was improper to target Americans.

But resentment against the United States has been growing among Palestinians.

In June, the U.S. government announced it had received "credible reports" of plans to kidnap U.S. citizens in Gaza.

An opinion poll released Wednesday, showed 97 percent of Palestinians believe U.S. policy favors Israel and 96 percent believe the U.S. commitment to establishing a Palestinian state was insincere. The poll of 1,318 people had a margin of error of 3 percentage points

A team of U.S. investigators who photographed the charred sports utility vehicle was pelted with rocks by about a dozen Palestinian youths as about 200 Palestinians looked on.

As the angry crowd chanted "Allahu Akbar" — "God is great" — the Americans rushed back into their cars, surrounded by nervous Palestinian security officers with rifles raised. Palestinian police beat some people in the crowd while pushing the spectators back, and the cars sped away under a hail of stones.

The U.N. representative to the Middle East, Terje Roed Larsen (search), said the attack was "an ominous widening of the conflict."

But Israeli and Palestinian analysts said the attack would likely give Israel an even freer hand to carry out raids in the West Bank and Gaza and targeted killings of militant leaders.

"It will allow Israel to move with greater determination in Gaza and in the West Bank to dismantle the terror capabilities," said Gerald Steinberg, an analyst at Israel's Bar Ilan University (search).

About six students and two scholars from Gaza come to the United States each year on Fulbright scholarships, said Barry Ballow, director of the Office of Academic Exchange Programs at the State Department.