President Bush said Friday that Islamic militants who beheaded U.S. hostage Paul Johnson (search) in Saudi Arabia (search) are "barbaric people" who are trying to intimidate America and shake its will.

Vice President Dick Cheney also condemned the killers: "They have no shame, not a shred of decency, and no mercy. America will hunt down the killers, one by one, and destroy them."

Bush learned of Johnson's death after a speech to troops at Fort Lewis (search), Wash.

"The murder of Paul shows the evil nature of the enemy we face," Bush said as he prepared to board Air Force One. "These are barbaric people. There's no justification whatsoever for his murder. And yet they killed him in cold blood.

"And it should remind us that we must pursue these people and bring them to justice before they hurt other Americans," the president said. "See, they're trying to intimidate America. They're trying to shake our will. They're trying to get us to retreat from the world. America will not retreat. America will not be intimidated by these kinds of extremist thugs. May God bless Paul Johnson."

A top Saudi Arabian official on Friday said his country will hunt down those responsible for killing Johnson, and that security forces were in a firefight with Al Qaeda militants in central Riyadh.

Adel al-Jubeir, foreign affairs adviser to the Saudi crown prince, told reporters that those militants may be involved in Johnson's death, but it is too soon to know. He said a number of militants had been killed.

"Once it is over we will have a better picture," he said.

Al-Jubeir said the Saudis deployed over 15,000 security personnel in the search for Johnson. He vowed to find and punish those responsible.

"We will leave no stone unturned to confront this evil and destroy it," he said.

Secretary of State Colin Powell described Johnson as an innocent individual who "was just trying to help people and trying to do his job."

If anything, he said Johnson's murder "will cause us and, I'm quite confident, it will cause our Saudi colleagues to redouble our efforts to go after terrorists wherever they are trying to hide."

U.S. officials said Abdulaziz Issa Abdul-Mohsin al-Muqran (search) is on the top of the list of possible militants responsible for the killing, though he may not have wielded the weapon.

al-Muqran, who is thought to lead a group known as Al Qaeda (search) in the Arabian Peninsula, is perhaps the most senior and experienced Al Qaeda operative in Saudi Arabia, believed to be involved in the May attacks on the housing compounds in Riyadh and other attacks it the kingdom, the officials said.

He is known to be an expert in bomb-making and explosives.