Paul Johnson, 49, of Stafford Township, N.J., was kidnapped Saturday by a group calling itself al Qaeda in the Arabian Peninsula. The organization is believed to be headed by al Qaeda's chief in the Saudi kingdom, Abdulaziz Issa Abdul-Mohsin al-Moqrin.
On Tuesday, an Islamic Web site showed a videotape of Johnson wearing a blindfold.
"My name is Paul Marshall Johnson, Jr.," says the hostage, seated and with an elaborate tattoo showing on his left shoulder. "I am an American. … I work on Apache helicopters."
He speaks for only 25 seconds. His words are slurred as he states his name, reports CBS News Correspondent Jim Stewart.
A U.S. official said the threat should be taken "very seriously" because the posting appeared to be credible and militants have used the site before. "It has been a good indicator in the past," the official said, speaking on the condition of anonymity.
"I think unless they find him and can rescue him right now, the militants are very likely to carry out their threat," Neil Livingstone, CAO of Global-Options and an expert on terrorism and hostage situations, told the CBS News Early Show.
The Web site was posted on the same day Saudi Arabia's ruling crown prince warned Islamic militants that the kingdom planned to deploy more security forces than they had ever faced before.
"Be assured that the kingdom has enough men whom you haven't seen so far, but within the coming few days you will see them," Crown Prince Abdullah told the militants, whose attacks have increased during the past three months.
The tape on the Web site showed a hooded man reading a statement and holding an AK-47 rifle. As the man was reading, a subtitle on the screen identified him as al-Moqrin.
His statement was similar to a printed message on the Web site that carried the name of al Qaeda in the Arabian Peninsula. It said the group gave Saudi authorities 72 hours — by Friday — to release "mujahadeen" militants or it would kill the hostage.
Segments of the tape appeared to have been edited together and showed a blindfolded Johnson sitting in a chair with his profile to the camera. In one sequence, Johnson appeared to have a bandage around his neck, or a gag that had been pulled down from his mouth.
The tape also displayed his Lockheed Martin identification card.
Adel al-Jubeir, foreign affairs adviser to the Saudi government, told The Associated Press that the kingdom was trying to determine the authenticity of the tape and would consult with the Bush administration about what to do next.
Al-Jubeir also said the current situation with the Islamic militants was not a crisis but a serious issue that the Saudi kingdom will be dealing with for some time.
"Their strategy is to try to sow fear in people's hearts, and to panic, and to cause an exodus of foreign workers from Saudi Arabia, in particular Westerners," he said.
"They are trying to scare foreign workers into leaving Saudi Arabia because they believe it will weaken the Saudi economy and consequently weaken the Saudi government, but they are mistaken."
Livingstone tells the Early Show 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.
Saudi security forces arrested a militant north of Riyadh on Tuesday as they stepped up presence in and around the city in a hunt for Johnson's kidnappers.
The militants previously threatened to treat Johnson as U.S. troops treated Iraqi detainees, a reference to the month-old controversy over sexual and other abuse at Abu Ghraib prison.
Members of Johnson's family, through a police officer stationed outside their home in Little Egg Harbor Township, N.J., declined an offer by CNN to view the video before it was aired. They could not immediately be reached for comment.
On Monday, Johnson's son spoke to reporters about his father's love of Arabic culture. Paul Johnson III said his father once sent a copy of the Koran to his sister, with passages highlighted from the Islamic holy text that he felt were especially important.
"He felt he never had any fear for his safety and respects and honors their traditions and cultures," Johnson III said. "Dad said many times he loved living in Saudi Arabia."
Westerners in Saudi Arabia are responding to the attacks by moving to high-security compounds or even to neighboring Bahrain, and by pushing for the right to armed private guards, according to diplomats and real estate agents.
Western embassies in Riyadh, the Saudi capital, are negotiating with the government for a relaxation of the ban on private security guards carrying firearms, a Western diplomat said, speaking on condition of anonymity.