Palestinian leaders' failure to clamp down on terrorists led to the deadly attack on a U.S. diplomatic convoy that killed three Americans in Gaza, President Bush said Wednesday.

Several hundred American citizens who live in the predominantly Palestinian area were advised by the U.S. Embassy in Tel Aviv (search) to depart.

Also, Secretary of State Colin Powell (search) pointedly told Palestinian Prime Minister Ahmed Qureia (search) when Qureia telephoned with condolences that the Bush administration couar to him 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," Powell said at a news conference.

Powell made clear to Israeli Foreign Minister Silvan Shalom as well as to Qureia, in separate telephone conversations, that they "need to move urgently to end terrorism," State Department spokesman Richard Boucher said earlier.

The U.S. Embassy's three-car convoy had just cleared a checkpoint and entered Gaza when an explosive device tore apart the second armored vehicle, killing the three Americans and wounding a fourth. They worked for a private security company "but they are not some outsiders," Boucher said.

An FBI team of investigators and forensic experts from Washington was traveling to the site to assist the Israelis with the investigation, Attorney General John Ashcroft (search) said.

Bush, bound for a conference with Asian leaders in Bangkok, Thailand, approved a statement sharply critical of the Palestinian leadership while he was flying to California to spend the night.

"Palestinian authorities should have acted long ago to fight terror in all its forms," the statement said. "The failure to create effective Palestinian security forces dedicated to fighting terror continues to cost lives. There must be an empowered prime minister who controls all Palestinian forces -- reforms that continue to be blocked by Yasser Arafat."

On the West Bank, Arafat condemned the attack as an "awful crime" and said he had ordered an investigation.

But Bush and other U.S. officials took no public notice of Arafat's remarks. Powell told Qureia that the United States expected "full cooperation in investigating this heinous act and in bringing these murders to justice," Boucher said.

State Department officials said they were asking the Israeli government to help evacuate any American who wanted to leave Gaza. The department also urged all U.S. citizens on the West Bank to take all take precautions against possible attacks.

Powell said the three Americans were "murdered by terrorists" on a mission of peace -- traveling with U.S. diplomats who intended to advise Palestinians about the Fulbright program for studies abroad.

These same terrorists "are killing the dreams of the Palestinian people," Powell said.

John Limbert, president of the American Foreign Service Association, the labor union of 23,000 active and retired foreign service personnel, said the attack "underlines the vulnerability of men and women of the U.S. Foreign Service who advance our nation's vital interests around the globe."

While the U.S. government cannot provide 100 percent protection, there must be sufficient funding to do as well as possible, Limbert said in a statement.

On Capitol Hill, Sen. Rick Santorum, R-Pa., said the attack was indication that Arafat "has not gotten serious about solving the problems that exist in the area controlled by the Palestinian Authority, and this process is not going to move forward until they do."

Boucher identified the three slain Americans as John Branchizio, 36; Mark T. Parson, 31; and John Martin Linde Jr., 30.