Within a week, three Westerners have been killed and another, an American defense worker, is being held hostage, reports CBS News Correspondent Richard Roth.
On Islamic Web sites, al Qaeda claimed responsibility for kidnapping American aeronautics engineer Paul Johnson, whose Lockheed-Martin business card was posted along with a warning: al Qaeda is seeking vengeance for the U.S. treatment of Muslim prisoners in Baghdad and Guantanamo Bay.
Statements attributed to al Qaeda also said the group was responsible for killing an American worker in Riyadh Saturday. Kenneth Scroggs was shot in the back outside his home.
Video posted on the Internet purports to show the murder last Tuesday of U.S. defense contractor Robert Jacobs. Al Qaeda claims this attack too, saying it wants to "cleanse" Saudi Arabia of infidels.
"They have a program, those militants - and they want to 'Talibanize' Saudi Arabia - and they will not be allowed to do so," says Jamal Khashoggi, a spokesman for the Saudi ambassador in London.
But the militants are clearly spreading fear in the oil kingdom. In the huge expatriate community, there are some 35,000 American citizens. The United States has urged them to leave.
"It's not unraveling, but it's certainly a dangerous situation right now," says Secretary of State Colin Powell. "Terrorists are going after the Saudi leadership, they're trying to make the country unstable. I know that the Saudis are treating it with the utmost seriousness and they're counter-attacking."
Crown Prince Abdullah made a public pledge the militants won't escape justice. However, since last month's killing of 22 foreigners at an oil industry complex, gunmen have also killed a BBC cameraman and critically wounded a correspondent. And the Saudis' most-wanted list is only getting longer.
The U.S. Embassy warned that the attacks appeared to follow extensive surveillance of the two Americans slain over the past week.
The killings "involved extensive planning and preparation," a U.S. Embassy warden message said. "Often, this pre-attack surveillance can be detected."
Al Qaeda, led by Saudi-born Osama bin Laden, often rails against Saudi Arabia's rulers for their close links to the United States.
Powell said Saudi leaders are mobilizing all resources against militants but added, "I think that there is more that they can do."
The Saudis can "build up their forces" and cut off funding for militants, he said in a broadcast interview. "There's probably more we can do with respect to intelligence exchange, and we are working at all of these," he said.
Prince Bandar, the Saudi Ambassador to the United States, called the attacks on Americans "craven acts of evil."
"Their intention is to shake our will, to frighten away our friends and allies, and to undermine our society," Bandar said.
Saudi security officials, speaking on condition of anonymity, said police stormed a suburban Riyadh house Sunday morning, arresting a man inside and confiscating a computer. It wasn't clear whether he was linked to any of the past week's shootings.
The purported al Qaeda statement, posted late Saturday on an Islamic Web site, threatened to treat Johnson, the abducted American, as U.S. troops treated Iraqi prisoners — a reference to sexual and other abuses at Abu Ghraib prison in Iraq.
The al Qaeda statement said Johnson is one of four experts in Saudi Arabia working on developing Apache attack helicopter systems.
"Everybody knows that these helicopters are used by the Americans, their Zionist allies and the apostates to kill Muslims, terrorizing them and displacing them in Palestine, Afghanistan and Iraq," said the statement. It said al Qaeda would release a videotape later with Johnson's confessions and its demands.
A car belonging to Johnson was found Saturday near Imam University, security officials said. Saudi press reports said the car was booby-trapped and later caught fire. The university is about 12 miles from the neighborhood where Scroggs was shot.
Lockheed Martin issued a statement confirmed that Johnson was missing. The U.S. Embassy said it was working with Saudi officials to find the kidnapped American.
Paul Johnson III, of Port. St. John, Fla., asked the kidnappers to let his father go.
"He doesn't deserve it. It's not his fault he's over there. It's his job," he said in a broadcast interview.
He said his father had been nervous about being in Saudi Arabia. "My dad's probably praying, wondering how he got himself into this and how he can get himself out," he said.
The militant attacks against Westerners, government targets and economic interests in the kingdom have surged despite a high-profile campaign against terrorists the government began after suicide bombings last year.
Terror experts have noted that the militants are using several tactics — including shootings and ambushes where the gunmen do not die — rather than limiting themselves to suicide bombings or swift attacks under the cover of darkness.
The statement claiming Saturday's shooting and kidnapping was signed by al Qaeda in the Arabian Peninsula, the same group that claimed responsibility for a May 29-30 shooting spree and hostage-taking in the eastern Saudi oil hub of Khobar that killed 22 people, most foreigners.
Saudi security arrested one attacker in Khobar, but three others escaped.
Saudi Arabia relies heavily on a foreign work force. An estimated 8.8 million foreigners work among 17 million Saudis in the kingdom, some in the oil sector, banking and other high-level businesses, but the majority in service-industry jobs such as maids, bellboys or taxi drivers.
The U.S. Embassy had already advised Americans to leave the kingdom, and the British Embassy on Sunday said it was authorizing the voluntary departure of nonessential staff and their families.
Meanwhile, several Islamic Web sites were carrying links to a videotape — also purportedly from al Qaeda — that claims to show last Tuesday's killing of Jacobs, 62, of Murphysboro, Ill., who worked for U.S. defense contractor Vinnell Corp.
The video, that claims to show last Tuesday's killing of Jacobs, 62, of Murphysboro, Ill., is less than two minutes long. It does not show any faces. It begins with men running in a garage and a voice yelling in English, "No, no, please!" A shot is fired, and the body of what appears to be a Western man falls to the ground. Two gunmen fire at least 10 more shots at the fallen man, then one kneels by his head and motions as if he is beheading him.