According to ABC News, the kidnappers of Paul Johnson:
"had help from sympathizers within the Saudi security forces, according to an account of the operation posted by the al-Qaida cell on an Islamic extremist Web site Sunday. The sympathizers gave police uniforms to the militants, who then snatched the American engineer at a fake checkpoint in the city, the posting said."
If this is true (you can't believe everything you read on Islamic extremist Web site), then it raises several possibilities:
1. The Saudi security forces are riddled with Islamic sympathizers.
2. The Saudi security forces infiltrated the Al Qaeda cell that kidnapped Johnson, perhaps even using Johnson as bait to capture or eliminate the cell. That explains why they managed to kill the cell leaders only hours after Johnson was beheaded.
3. All of the above.