Terrorists holding Paul M. Johnson Jr. threatened Tuesday to kill him in three days unless Saudi authorities release al Qaeda prisoners. They did not give a specific time. The threats were made on a video shown on an Islamic Web site following Johnson's abduction over the weekend.
A U.S. official said the threat on the tape should be taken "very seriously" because the posting appeared to be credible and militants have used the site before.
Late Wednesday, Saudi security forces and police surrounded a house in Riyadh, but withdrew hours later, witnesses said. Security officials would not comment on the operation.
Witnesses said they heard gunfire and the anti-terrorism force use loudspeakers to urge those inside the house to surrender.
It was unclear whether anyone was in the house at the time. The witnesses said security forces later searched the house and a nearby mosque before the large force withdrew from the area.
On Tuesday, Saudi forces arrested two militants north of Riyadh as they increased security while hunting for Johnson's kidnappers.
Officials in Washington said U.S. and Saudi authorities had teamed to find Johnson.
"We have people and resources on the ground in Saudi Arabia," State Department spokesman Richard Boucher said. "They've been in close touch with their Saudi counterparts on this matter."
He said Saudis "have the lead on this and we work with them."
But U.S. Sen. Frank Lautenberg said he was discouraged after an hour-long meeting at the Capitol with the Saudi government's top foreign policy advisor, Adel al-Jubeir.
"They're at a loss. He says they're using every resource that they have to try to free him," said Lautenberg, D-N.J. "Right now the prospects are gloomy."
Johnson's son appeared Wednesday on CNN to implore the Saudi government to find his father.
"I plead with the Saudi government and the group of men that are holding my father to please let him return home safely," Paul Johnson III said. "He will leave your country. You will never see him again. I just plead with them to get him home safely."
A candlelight vigil for the hostage was planned for Thursday night behind a firehouse in this rural community about 20 miles north of Atlantic City.
"We all hope Paul comes back," Dan Pomponio, a neighbor of Johnson's sister in Little Egg Harbor, said Wednesday. "You can only cross your fingers and hope."
As friends here Wednesday remembered the clean-cut young man who liked fast cars and motorcycles, family members described Paul Johnson Jr. as "an honorable man" who respected Saudi Arabia's culture and traditions. He "always felt safe in Saudi Arabia" and never feared living there, said his sister, Donna Mayeux.
Johnson, 49, moved to Florida in the early 1980s to work for Lockheed Martin, and had worked in Saudi Arabia for more than a decade. At the time of his abduction, he had been working on targeting and night vision systems for Apache helicopters.
Neil Livingstone, CAO of Global-Options and an expert on terrorism and hostage situations, told the CBS News Early Show on Wednesday that the Saudis "are probably combing every section of the city that they think they can find him in, every place they think they might be able to locate him."
"That said, the Saudis haven't been very effective in the past at finding the terrorists," he said. The goal of the ultimatum is likely "to show that the Saudi government is incapable of really protecting Americans or even its own citizens in that country."
The day Johnson was seized, Islamic militants shot and killed Kenneth Scroggs, from Laconia, N.H., in his garage. Scroggs was the third Westerner killed in a week, after the shooting death of an Irish cameraman for the British Broadcasting Corp. on June 6 and another American who was also killed in his garage June 8.
A videotape emerged this week of another American hostage, Robert Jacobs, who was apparently shot to death by Saudi insurgents, CBS News Correspondent Shelia MacVicar reports.