Al Qaeda said on Sunday it killed one American and kidnapped another in Saudi Arabia to "avenge US mistreatment" of Muslim prisoners in Iraq and Guantanamo Bay in Cuba, according to an Islamic website.
The site also posted a video it said showed the killing of another American in the capital Riyadh earlier in the week.
A string of attacks on Westerners has raised security fears in the world's top oil exporter.
Witnesses said the American was shot on Saturday as he parked his car in front of his villa in a Riyadh suburb.
The "al-Qaeda Organisation in the Arabian Peninsula" said in a statement on the Sawt al Jihad Islamic website that the man kidnapped at the same time was Paul Marshal Johnson, a 49-year-old New Jersey engineer specialising in Apache helicopters, whom it branded "an American Christian parasite".
"The Mujahideen were able in the same operation to kill another American working as a manager in the military sector. They stalked him and then they killed him in his home," it said.
It said it would soon release a video of the captive, whose business card showed he worked for Lockheed Martin as a site manager and systems engineer.
"(We) reserve the legitimate right to deal with the Americans in the same way to avenge what the Americans did to our brothers in (Baghdad's) Abu Ghraib prison and Guantanamo," it said.
The US embassy confirmed one American was shot dead in Riyadh on Saturday and that another was missing.
"We are working with local authorities to find him," a spokeswoman said.
Police also found a car rigged with explosives in a suburb near foreigners' housing, Saudi-owned Al Arabiya television said, but it was unclear if the incidents were linked.
Al-Qaeda in Saudi Arabia is led by Abdulaziz al-Muqrin, who has vowed to make 2004 "a miserable and bloody year" for the ruling family and to drive out foreigners from the kingdom, the birthplace of Islam.
An estimated six million foreigners work in Saudi Arabia, including 35,000 Americans and 30,000 Britons.
The kingdom has been fighting militants for a year, arresting and killing many. Riyadh says the militants are going for soft targets after the clampdown.