The three Americans killed or kidnapped by Islamic radicals in Saudi Arabia in the past week were likely selected as targets many days or weeks in advance and singled out because of their work as military contractors, U.S. and Saudi officials said Sunday.

Authorities continued to search for the kidnapped American, Paul M. Johnson Jr., 55, an employee of Bethesda-based Lockheed Martin Corp., whose family reported Saturday that he had vanished in Riyadh, the Saudi capital.

A group calling itself Al Qaeda of the Arabian Peninsula issued a statement Saturday saying it had captured Johnson and would treat him in the same way that U.S. troops treated Iraqi detainees in the Abu Ghraib prison outside of Baghdad and the Guantanamo Bay detention facility in Cuba.

Although Johnson was employed by Lockheed Martin, the telephone number on his business card indicated that he worked at the Riyadh headquarters of Advanced Electronics Co., a Saudi technology firm that manages a number of defense contracts for the Saudi government.

Advanced Electronics was the employer of Kenneth Scroggs, another American, who was gunned down by three assailants as he pulled into the garage of his Riyadh home Saturday afternoon, Saudi officials said.

A third American was fatally shot in his Riyadh home on Tuesday after leaving the Riyadh office of Vinnell Corp., a Fairfax-based subsidiary of Northrop Grumman Inc. Robert C. Jacobs, 62, worked for Vinnell on a project to train the Saudi National Guard. Seven Vinnell personnel were killed in May 2003 in a suicide bombing of a residential compound for Westerners in Riyadh.

On Sunday, the U.S. Embassy in Riyadh warned Americans in the kingdom to pay close attention to their surroundings and to avoid predictable workday routines that could make them easy targets. The embassy statement said last week's attacks on Americans "appear to have involved extensive planning and preparation and were likely preceded by extensive pre-attack surveillance."

In its statement Saturday, the al Qaeda-affiliated group said Johnson was one of four experts in Saudi Arabia on the Apache attack helicopters used by the U.S. military elsewhere in the Middle East. The statement indicated that Scroggs also advised the Saudi government on the use of Apaches.

Advanced Electronics, located in an industrial park near King Khalid International Airport outside Riyadh, was awarded a five-year, $10 million U.S. Army contract in 1999 for repair work on Apache systems. The program was scheduled to expire in March, according to a contract announcement issued at the time.

It was unclear whether Scroggs had been involved in Apache work.

Executives at the firm declined to be interviewed Sunday, but released a statement confirming that a U.S. employee had been killed "at the door of his house" in Riyadh on Saturday. The firm did not identify Scroggs by name, but called him "a very serious and sincere employee of the company for over 12 years."

In a statement to its own employees, Lockheed Martin said that Johnson had worked in Saudi Arabia on the Apache program, specializing in a targeting system known as Target Acquisition and Designation Sites/Pilot Night Vision System. Known as the "eyes of the Apache," it enables the helicopter's pilots to fly at low altitudes in the dark and in bad weather.

Lockheed Martin, the Pentagon's largest contractor, began evacuating its employees' dependents from Saudi Arabia in mid-April after the State Department issued a strongly worded warning urging Americans to leave the country.

The company declined to disclose how many of its employees were stationed in Saudi Arabia or what security measures were being provided to them. "As courageous and brave as they are, they go over there as volunteers," said Tom Jurkowsky, a Lockheed spokesman. "Security is paramount, we're aware of the warnings, the intelligence provided by the embassy. We take necessary precautions."

Lockheed also declined to comment on its work in the kingdom, citing security concerns. Lockheed manages international aircraft depots in Saudi Arabia, according to a Securities and Exchange Commission filing. Advanced Electronics has a contract with Lockheed to provide electronics for the F-16 fighter jet, according to Advanced Electronics' Web site.

Secretary of State Colin L. Powell said he was satisfied with the Saudi government's efforts to investigate the recent attacks and to prevent future bloodshed. "The Saudis know that this is an enemy that is coming after them," he said on "Fox News Sunday." "The killing of foreigners, whether they're Americans or Brits, or what are they, is a direct attack against the Saudi regime."

Mohsen Awajy, a Saudi lawyer and former Islamic radical who now advises the government on dealing with militants, said al Qaeda cells were targeting individual Westerners involved with the military in a bid to regain popular sympathy in Saudi Arabia. He said many Saudis were appalled by recent al Qaeda bombings that resulted in the deaths of Muslims and of expatriates who were seen as important cogs in the country's economy.

"The militants are trying to show some justification for what they're doing," he said. "They are also trying to choose the easiest targets because they are finding it harder and harder to do anything on a bigger scale."

The recent attacks are evidence that for contractors, "the risks are much higher than people anticipated," said Peter Singer, foreign policy fellow at the Brookings Institution. "Basically, if companies are going to keep people in Saudi, they are going to have to provide better security guarantees. The pay is going to have to reflect the higher danger."

On Sunday, Lockheed's main Web page was dedicated to the kidnapping. "Our thoughts and prayers are with Paul M. Johnson, Jr. and his family," the Web site said.

Whitlock reported from London, Merle from Washington.