RIYADH, Saudi Arabia - An al-Qaida cell fulfilled its threat to kill an American hostage, beheading him and showing the grisly photos on the Internet. Saudi officials claimed they later gunned down four militants including the cell leader who allegedly masterminded Paul M. Johnson's kidnapping.

State-run television showed pictures of the four slain militants' bodies Saturday. Among them was one it said was Abdulaziz al-Moqrin, the reputed leader of al-Qaida in Saudi Arabia and Saudi Arabia's most-wanted terror suspect.

Hours earlier, a message on an Islamic militant Web site said the reports of al-Moqrin's slaying were false. The message could not immediately be verified, but it appeared on a Web site that has had similar messages in the past.

A U.S. official who spoke on condition of anonymity Friday had confirmed al-Moqrin's killing, while a Saudi official had said forensic tests would confirm its identity.

Johnson, who had worked in Saudi Arabia for more than a decade, was the latest victim of an escalating campaign of violence against Westerners that aims to drive foreign workers from the kingdom and undermine the ruling royal family, hated by al-Qaida. Osama bin Laden, al-Qaida's leader, is a Saudi exile.Johnson's severed head was shown on a Web site Friday. The photographs and a statement, in the name of Fallujah Brigade of al-Qaida in the Arabian Peninsula, appeared after Johnson's wife went on Arab television and tearfully pleaded for his release."In answer to what we promised ... to kill the hostage Paul Marshall (Johnson) after the period is over ... the infidel got his fair treatment," the al-Qaida statement said. "Let him taste something of what Muslims have long tasted from Apache helicopter fire and missiles."President Bush condemned the beheading and vowed that "America will not be intimidated by these kinds of extremist thugs."After news of Johnson's death was released, a Saudi security official said a witness took note of the license number of a car from which his body had been dumped just outside Riyadh Friday and informed police.Police stopped the car at a gas station in central Riyadh and a shootout ensued in which al-Moqrin was killed with three other militants, Saudi officials said in the television broadcast Saturday."Security forces managed last night in confrontations with a group of terrorists to kill four of them, the top being Abdulaziz Issa Abdul-Mohsin al-Moqrin, who claims to be the leader of the gang that condemns people as infidels," the state-run TV announcer said, reading from a statement attributed to an unidentified spokesman for the Interior Ministry.The photograph said to be of al-Moqrin showed the face of a young man, clean-shaven except for his mustache. It resembled earlier known photographs of him. A trickle of blood ran from the mouth of another of the militants pictured.Saudi officials in Washington said on condition of anonymity that five Saudi security officers were also killed in the gunbattle. Two suspects escaped, said one Saudi security official who took part in the raid.Saturday's Interior Ministry statement also said authorities confiscated three cars used by the cell, including one believed to be used in the June 6 killing of Irish cameraman Simon Cumbers.Forged identity papers and a large amount of weapons also were confiscated, including three rocket-propelled grenade launchers, hand grenades and automatic rifles, the statement said.The killing of al-Moqrin, 31, would be a coup for the Saudi government, which has been under intense pressure to halt a wave of attacks against Westerners in the kingdom.The airing of the pictures appeared to be a direct rebuttal of the Web posting that appeared Saturday denying al-Moqrin was killed."Some satellite networks and news agencies have been propagating the false news that Abdulaziz al-Moqrin, God preserve him, has been killed," that statement said. "We would like to say that such claims, unleashed by the tyrants of Saudi Arabia, are aimed at dissuading the holy warriors and crushing their spirits."The statement denying al-Moqrin's death was signed by his group and said another message wound appear soon.Johnson, 49, who worked on Apache attack helicopter systems for Lockheed Martin, was kidnapped last weekend by militants who threatened to kill him by Friday if the kingdom did not release its al-Qaida prisoners. The Saudi government rejected the demands.One of three photographs posted on the Web site showed a man's head, face toward the camera, being held by a hand. The two others showed a beheaded body lying prone on a bed, with the severed head placed in the small of his back, the clothes underneath bloodied. One showed a bloody knife resting on the face.The beheaded body was dressed in a bright orange jumpsuit, similar to one Johnson is seen wearing in earlier videos released by the kidnappers."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 al-Qaida statement said.There are 35,000 Americans among the millions of Westerners who work in Saudi Arabia.Soon after the statement appeared, a number of Web sites that had links to it became inaccessible, with messages saying they were closed for maintenance.Johnson's beheading is the latest in a new, more dramatic wave of terror attacks for Saudi Arabia: bodies dragged on streets, traffic police blown up in their offices, hotel guests taken hostage and a chef shot outside an ATM machine. The attacks have killed dozens of people, mostly foreigners, over the past two months.The violence is escalating despite an aggressive campaign by the government to root out terrorism, leaving many wondering whether the attacks are just the beginning or - as the government continues to insist - the last gasps of a desperate group reacting to the pressure of the hunt.Johnson was seized on June 12, the same day that Islamic militants shot and killed Kenneth Scroggs of Laconia, New Hampshire, in his garage in Riyadh.Scroggs worked for Advanced Electronics Co., a Saudi firm whose Web site lists Lockheed Martin among its customers. The office number on Johnson's business card was for Advanced Electronics.The same week as Scroggs' death, militants shot and killed another American, Robert Jacobs, and Cumbers in Riyadh.