The U.S. State Department said one American was among the 22 people killed during the spree and Western diplomatic sources said a British citizen also died. The Saudi Interior ministry confirmed those reports and said the death toll also included citizens of Saudi Arabia, Italy, India, the Philippines, Sri Lanka, South Africa and Egypt.

On Saturday, the terrorists attacked both the Arab Petroleum Investment Corporation's compound and the Al-Khobar Petroleum Center. Employees of Royal Dutch/Shell ( RD), Honeywell ( HON) and General Electric ( GE) lived in one of the compounds. Oil industry companies with offices in the compounds include a joint venture among Shell, Total SA ( TOT) and Saudi Aramco, Russia's Lukoil and China Petroleum & Chemical, or Sinopec. At least 11 people were reportedly killed in the initial attack.

Following the attack on the offices, the militants took hostages at the nearby Oasis Residential Resorts complex. After little activity overnight, three helicopters carrying Saudi security forces arrived shortly after sunrise Sunday morning in the kingdom.

The weekend attack is the second on oil assets in the Saudi Kingdom this month. On May 1, militants killed five foreigners in an attack on a petrochemical site in the Red Sea town of Yanbu. The earlier attack was followed by a sharp rise in crude prices.

Security concerns in the oil-rich Persian Gulf region have added to market worries about supply ahead of the peak summer driving season in the U.S. and Europe. Prices have repeatedly hit record highs on the Nymex and have twice closed within $42 a barrel.

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