Al Qaeda militants say they were helped by Saudi forces
Al-Muqrin claimed responsibility for Johnson's killing.
Al-Muqrin was extremely vicious, U.S. ambassador says.
The life and death of al Qaeda leader Abdel Aziz al-Muqrin.
Family and friends devastated by the killing.
RIYADH, Saudi Arabia (CNN) -- Al Qaeda militants who kidnapped and killed American engineer Paul Johnson said Sunday on an Islamist Web site that sympathetic Saudi security forces aided their kidnapping operation with police uniforms and vehicles.
Johnson, an employee of Lockheed Martin, was kidnapped June 12.
After a 72-hour deadline passed without the release of all al Qaeda prisoners and the departure of all Westerners from the kingdom, photographs of Johnson's head and body were posted on the Web site.
Hours later on Friday, Saudi security forces killed cell leader Abdel Aziz al-Muqrin and three others and captured 12 other suspected members of the cell.
In a lengthy narrative about the kidnapping that was posted Sunday on the site, the kidnappers said they stopped Johnson's car at a fake checkpoint, transferred him to another car and took him to another location.
But Saudi foreign policy adviser Adel al-Jubeir told CNN it was "in the realm of fiction" that Saudi security personnel cooperated with the militants.
"It's very easy to obtain police uniforms, military uniforms," he told CNN's "Late Edition." "You go to a surplus store, and you get all you want."
The kingdom's interior ministry, the home of its internal security forces, "is on the forefront of the war against terror," al-Jubeir said.
"The notion that our security services are infiltrated by the terrorists really doesn't hold," he said. "If that were the case, they would not be going after soft targets. They would be going after government installations."
Also Sunday, the Web site announced that Saleh al-Oufi, a former prison guard who is No. 5 on Saudi Arabia's list of most-wanted terrorists, would replace al-Muqrin as cell leader.
That announcement came less than 24 hours after the Web site denied Saudi reports that al-Muqrin was dead.
The al Qaeda cell and Saudi officials identified the other three militants killed as Faisal al-Dakhil -- No. 11 on Saudi Arabia's list -- Turki al-Muteiri and Ibrahim al-Durayhim. A Saudi security officer was killed and two were wounded in the operations, al-Jubeir said.
Al-Jubeir said incidents like Johnson's killing would not weaken Saudi Arabia's commitment to "go after" terrorist elements.
"They believe that if foreigners leave Saudi Arabia, and in particular Americans and other westerners, that our economy will be crippled and our government will be weakened," he said. "It is a difficult time, but it is a manageable time. We believe that we still have control over safety in Saudi Arabia."
"We will be very vigilant in trying to ensure the safety of everybody in the kingdom," he said. "And we will be merciless when we go after the terrorists who try to wreak havoc on our society."
Critics have accused Saudi Arabia's monarchy of giving financial support to terrorists, but a report issued last week by the U.S. independent commission on the September 11, 2001, terrorist attacks found no evidence of such support, a finding that al-Jubeir said "vindicated" his country.
Two members of the commission said Sunday that Saudi Arabia, along with Pakistan, had passively supported the activities of terrorists within their borders by failing to act against them, but added that that no longer appeared to be the case.
"That era is over," said former Navy Secretary John Lehman. "They now recognize the threats, and I think they are cooperating with us." (Full story)
Lehman and fellow commission member Richard Ben-Veniste each noted, however, that some Islamic schools -- madrassas -- still pose a problem.
"The history of providing support for the madrassas -- in which children are taught to hate those who do not share their common beliefs and that it is acceptable to attack, in violent forms rather than in discourses, differences in philosophies, culture and religion -- has been a principle source of worldwide unrest and support of elements hostile to Western ideas and civilization," Ben-Veniste.
"We are hopeful that now that the Saudis in particular have seen the results of these years of support of this kind of a movement, that they will now move to change what has been in place for so long."