Saudi Arabia's security forces helped the al Qaeda cell that kidnapped slain American Paul Johnson, the group has claimed.
An al Qaeda website said supporters within the service supplied the group with police uniforms and helped it set up a fake checkpoint.
The West has long feared that al Qaeda sympathisers within the police and security forces were aiding attacks on expats in the Kingdom.
The article said militants wearing police uniforms and using police cars set up a fake checkpoint on al-Khadma Road near Imam Mohammed bin Saud University.
They pulled over Mr Johnson's, car anaesthetised him and carried him to another car, the article said.
Mr Johnson's car was then blown up.
"A number of the co-operators who are sincere to their religion in the security apparatus donated those clothes and the police cars," the article said.
"We ask God to reward them and that they use their energy to serve Islam and the mujahedeen."
Mr Johnson, a defence contractor who worked on Apache helicopters, was beheaded after demands to release al Qaeda prisoners in Saudi Arabia were not carried out.
Abdulaziz al-Moqrin, al Qaeda's leader in Saudi Arabia, was later killed by Saudi security forces.
In a separate article written by Moqrin on the website he defends the beheading.
"Do those people want to see this infidel carry on the killing of the children and the raping of the women in Baghdad and Kabul in order to bless his killing?"
Westerners working in Saudi Arabia have complained that compound guards may have been complicit in al Qaeda strikes on housing complexes.
Western governments have also warned the Saudi royal family they need to do more to root out al Qaeda sympathisers in the police and security forces.
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