Islamic radicals in Saudi Arabia beheaded an American contractor whom they kidnapped six days earlier, and in a defiant statement on the Internet pledged more attacks on Americans. Hours later, Saudi security forces reported they had shot dead the leader of the group that asserted responsibility for the abduction.

The decapitated corpse of Paul M. Johnson Jr., an employee of Bethesda-based Lockheed Martin Corp., was found on the outskirts of the Saudi capital, Riyadh, on Friday, after the group calling itself al Qaeda of the Arabian Peninsula announced his death and posted photos of his remains on a Web site.

Johnson was the third American in less than two weeks to meet a violent death in Riyadh. His kidnapping, the first of a Westerner in Saudi Arabia, and the manner of his killing marked a new kind of assault on the American presence in the oil-producing kingdom. "The infidel got his fair treatment," the group said in a statement.

Radicals say that vengeance for the occupation of Iraq is part of the reason for the campaign, which has led many Americans to leave the country and made those who remain fearful of streets they once found safe. The ultimate goals, the groups say, are to drive foreigners out of the kingdom and overthrow the House of Saud royal family.

Saudi officials reported that one of three men killed in a shootout with security forces about an hour after the announcement of Johnson's death appeared to be Abdulaziz Muqrin, leader of the al Qaeda group. The Saudi government has said that Muqrin's death or capture would be key to suppressing the group; DNA testing was planned.

President Bush denounced the killers as "barbaric people" trying to intimidate Americans. "The murder of Paul shows the evil nature of the enemy we face," Bush said in Fort Lewis, Wash. "They're trying to shake our will. They're trying to get us to retreat from the world. . . . America will not be intimidated by these kinds of extremist thugs."

Johnson was reported missing by his family last Saturday, the same day that another American contractor who worked out of Johnson's office, Kenneth Scroggs, was killed by gunmen as he drove into his garage in Riyadh.

Last weekend, the group announced it was holding Johnson and said he would be treated as Muslim detainees were treated in Iraq's Abu Ghraib prison, scene of abuse by U.S. jailers, and Guantanamo Bay.

On Tuesday, Muqrin's group released a short video showing a blindfolded Johnson. A masked man, identified in a caption as Muqrin, threatened to execute him within 72 hours unless Saudi officials released an unspecified number of fellow jihadists from prison. The Saudi government refused and launched an effort to find and rescue Johnson.

According to Saudi spokesmen, the operation drew on 15,000 security personnel, including fire departments familiar with streets and neighborhoods. More than 1,200 homes had been searched by Thursday night.

In addition, the U.S. government dispatched 20 FBI agents to Saudi Arabia to assist in the search. The presence of American agents on Saudi soil was a sensitive subject for Saudi government officials, who played down the U.S. role and bristled at the suggestion that they were unable to control the insurgency.

Prince Nayef, the Saudi interior minister, said in an interview with the French newspaper Le Figaro that the Saudi government did not need outside assistance. He noted that the United States has had difficulty finding terrorist suspects elsewhere, singling out Abu Musab Zarqawi, a radical who has eluded capture in Iraq despite a $25 million reward offer. Zarqawi has claimed responsibility for beheading another American, Nicholas Berg, in a kidnapping in Iraq this year.

In the days leading up to the deadline, Johnson's relatives and friends made emotional appeals on television for his release, telling the kidnappers that he was a friend of Muslims and deeply interested in their culture.

"Please release my father," Johnson's son, Paul Johnson III, told al-Arabiya television a few hours before the station reported the execution. "He is an innocent man. He loves Muslims. Saudi Arabia was his home." Johnson's wife, Thanom, made a similar plea in television interviews.

The kidnappers posted a statement on an Islamic Web site announcing his death and included three photos of his remains.

Johnson worked on Apache attack helicopter systems for the Saudi government. His kidnappers said he was singled out for that reason. "Let him taste something of what Muslims have long tasted from Apache helicopter fire and missiles," they said in a statement posted on the site. "The infidel got his fair treatment."

"To the Americans and whoever is their ally in the infidel and criminal world and their allies in the war against Islam, this action is punishment to them and a lesson for them to know that whoever steps foot in our country, this decisive action will be his fate," the statement said.

James C. Oberwetter, the U.S. ambassador to Saudi Arabia, confirmed Johnson's death. "The inhumanity of the crime exceeds all boundaries of civilized peoples," he said in a statement.

On Thursday, the U.S. Embassy in Riyadh issued a fresh warning to Americans in Saudi Arabia, saying that there was "credible information indicating that extremists are planning further attacks against U.S. and Western interests." The warning added that Americans living in private residences -- as opposed to the guarded compounds that house many expatriates -- were being "specifically targeted."

On April 15, the State Department urged all Americans to leave the kingdom and has ordered the evacuation of all non-essential embassy personnel. For the past two months, all Americans remaining in Saudi Arabia have been asked to register their presence with the embassy.

American workers, most of them with the oil industry, and dependents used to number close to 35,000. Embassy officials said they did not know how many had left.

Lockheed began evacuating employees' family members from the country in April after the State Department issued its warning. Lockheed, the Pentagon's largest contractor, declined to say how many of its workers remained in Saudi Arabia or to discuss what security precautions it was taking.

"Paul bravely carried out his duties, and the news of his sudden loss is a shock to everyone in the Lockheed Martin family," the firm's two top executives, Chairman and CEO Vance Coffman and President Robert J. Stevens, said in a memo to employees. "We grieve along with his family."

Lockheed employees have sent 700 e-mails and letters to Johnson's family in the last week, company spokesman Tom Jurkowsky said, adding: "It's a different mood around here. It's numbing."

Mustafa Alani, a Middle East security analyst at the Royal United Services Institute in London, said more kidnappings were likely.

"This sort of operation is easy to execute," he said in a telephone interview. "It's also cheap and very effective. How hard is it to shoot or grab somebody on the street? Look at all this television coverage they've gotten in the U.S. They've now discovered this cheap and easy way to get publicity."

Staff writer Renae Merle in Washington contributed to this report.