An Islamist website has posted a purported al-Qaida statement claiming responsibility for kidnapping one American and killing another Saturday in Saudi Arabia.
The statement says the group will treat the hostage the way Americans treated prisoners at Guantanamo and Iraq's Abu Ghraib prison.
The U.S. Embassy in Riyadh says it working with Saudi authorities to find the missing man, whose photograph was shown on the website.
Earlier, militants killed an American in Riyadh in the third fatal attack against Westerners there in a week.
The U.S. State Department has recently urged Americans to leave Saudi Arabia, and on Saturday, U.S. Ambassador James Oberwetter warned Americans who stay to exercise caution. He also praised Saudi Arabia's determination to fight terrorism.
Information for this report is provided by AP and Reuters.