Police cars and armored vehicles flooded the al-Malaz neighborhood in the Saudi capital Sunday as security forces surrounded a house where suspected militants were believed to have taken refuge after a shootout with police.

The massive operation was underway in the same district that was the focus of a huge security sweep against militants sought in the beheading of American hostage Paul M. Johnson Jr., whose body has still not been found.

According to an account of the kidnapping posted by a purported al Qaeda cell on an Islamic extremist Web site Sunday, Johnson's kidnappers had help from sympathizers within the Saudi security forces. The sympathizers gave police uniforms to the militants, who then snatched the American engineer at a fake checkpoint in the city, the posting asserted.

The account reinforced fears that some diplomats and Westerners in the kingdom have expressed -- that militants have infiltrated Saudi security forces, a possibility that Saudi officials have denied.

Saudi Arabia's ruler, King Fahd, vowed that militants in the kingdom would be stopped.

"The perpetrators of these attacks aimed at shaking stability and crippling security -- and it is a far-fetched aim, God willing," he said in a speech Sunday to the advisory Shura Council, according to the state-run Saudi Press Agency. "We will not allow this destructive bunch, led by deviant thought, to harm the security of this nation or affect its stability," the king, who has been incapacitated for years by a stroke, was quoted as saying.

Police barricaded off the al-Malaz district, where security forces surrounded the house. Witnesses said they had seen shooting between suspects and police before some men fled on foot, seeking refuge in the house.

It was the same area where Abdulaziz Muqrin, a key suspect in the kidnapping who was believed to be the leader of the reputed al Qaeda cell in Saudi Arabia, and three other militants were killed in a shootout with Saudi security forces Friday, hours after Johnson was killed and photos of his body and severed head were posted on a Web site.

The foreign policy adviser of Crown Prince Abdullah in Washington, Adel Jubeir, said Saudi officials were still looking for Johnson's body. "We are still combing through neighborhoods. And we hope that eventually we'll find the body and restore it to his family," he said on CNN's "Late Edition."

According to the Web site account of Johnson's kidnapping, militants wearing police uniforms and using police cars set up a checkpoint June 12 on a road leading to the airport, near Imam Mohammed bin Saud University.

"A number of the cooperators who are sincere to their religion in the security apparatus donated those clothes and the police cars. We ask God to reward them and that they use their energy to serve Islam and the mujaheddin," the Internet posting said.

When Johnson's car approached the checkpoint, the militants stopped his car, anesthetized him and carried him to another vehicle, according to the account.

In a separate posting on the Web site, Muqrin sought to justify the killing of Johnson, pointing to his work on Apache attack helicopters for Lockheed Martin.

Johnson "works for military aviation and he belongs to the American army, which kills, tortures and harms Muslims everywhere, which supports enemies in Palestine, Philippines, Kashmir," the posting said.

The articles appeared in Sawt al-Jihad, or Voice of the Holy War, a semimonthly Internet periodical said to be posted by the group that calls itself al Qaeda in the Arabian Peninsula.

The account of Johnson's kidnapping said the militants decided to behead him when Jubeir declared that Saudi Arabia would not negotiate with the kidnappers.

Asked about that, Jubeir said Sunday on CNN: "We have never negotiated with terrorists. We don't intend to do so. I believe what the al Qaeda people were trying to do is trying to justify a murder that is unjustifiable under any faith or under any principle of humanity."

A man checks damage to a car at the site of a clash between Saudi police and suspected members of a reputed al Qaeda cell in Riyadh, the capital.