RIYADH, Saudi Arabia -- A letter signed "The Believer" urges militants to spare American hostage Paul M. Johnson Jr., saying killing him would violate Islamic law. "I will curse you in all my prayers" if he is harmed, it warned.
The letter was posted late Wednesday on Web sites where al-Qaida supporters and other militants leave messages, and aired on a Saudi-owned television network, al-Arabiya.
Meanwhile, a senior Saudi official in the United States directly familiar with the investigation said Thursday night that U.S. and Saudi officials have had few promising leads in their search for Johnson, who was kidnapped Saturday by a group calling itself al-Qaida in the Arabian Peninsula.
The Saudi official said there had been no communications from the kidnappers except for a video and written statement posted Tuesday on a Web site. In the statement, Johnson's captors threatened to kill Johnson if al-Qaida prisoners in Saudi Arabia were not freed within 72 hours. The 72 hours ends sometime today.
The letter Wednesday, signed by Saad al-Mu'men -- a pseudonym meaning "Saad The Believer," -- identified the writer as a Saudi friend of Johnson's and said he had bestowed his protection as a Muslim on the American hostage.
It pointed to a saying by Islam's Prophet Muhammad: "If they were granted [Muslim] protection, then killing or taking their money or harming them is forbidden."
The Saudi official in the United States, who spoke to The Associated Press only on condition of anonymity because of the sensitive nature of the probe, said the FBI had sent a team of about 20 specialists in hostage rescue, hostage negotiations, profiling and other specialties who were working directly with Saudi officials.
More than 15,000 Saudi officers have been conducting a citywide search of Riyadh, going door-to-door in some neighborhoods considered hotbeds for terrorist sympathies and conducting surveillance in other parts of the city.
"We are even using the fire department, for instance, because they have knowledge of their neighborhoods, and districts," the official said. More than 1,200 Saudi homes had been searched as of Thursday night, the official said.
The Saudi official also said the chief suspect in Johnson's kidnapping, al-Moqrin, is also the main suspect in the shootings of a German citizen and an American in the kingdom recently.
Johnson, 49, had worked in Saudi Arabia for more than a decade.
The official said the Saudi government has directly communicated with Johnson's son in New Jersey to apprise him of the full extent of the search.
The son, Paul Johnson III, made a plea Thursday for his father's safe return, telling AP: "I want my father home."
In an interview on NBC's "Today" show, Johnson said he had received no indication that authorities have made any progress in the search for his father.
Security has been increased in and around Riyadh as the hunt for Johnson continues.
In the letter, al-Mu'men said Johnson -- who works for Lockheed Martin on targeting and night vision systems for Apache helicopters -- had expressed opposition to U.S. foreign policy and an interest in converting to Islam.
"He was interested in reading Quran translations ... I swear to God that once he said to me in public -- in a popular restaurant in Riyadh: 'I hate my country's politics. I am interested in Islam. If I convert, I will go and live with my wife in East Asia," al-Mu'men wrote. Johnson's wife is Thai.
Al-Mu'men said he was a colleague of Johnson but did not give any details about his job. He said he often invited Johnson for dinner and gave the American books on Islam as presents.
The letter was posted on Web sites know for Islamic extremist writings and was subsequently posted on the Web site of the Saudi-owned satellite television channel Al-Arabiya, http://www.alarabiya.net.
Al-Arabiya said that al-Mu'men said in an interview that he chose Islamic Web sites known for airing radical views because he was sure the kidnappers would monitor them.
Other contributors to the sites where the letter was posted ridiculed the message and called for Johnson's death.
Al-Arabiya told the AP that al-Mu'men had contacted the station but agreed to be identified only by the pseudonym. The station said it had confirmed the man it interviewed was the author of the letter, but it did not say how.
Al-Mu'men refused an interview with the AP in a message relayed through the station.
Amid concern over Johnson's fate, the U.S. State Department updated a 2-month-old travel warning for Saudi Arabia, pointing out that attacks on Americans there have resulted in deaths and injuries and, in a reference to Johnson, a kidnapping.
A department statement reminded American citizens of "the continuing serious threat to their safety while in Saudi Arabia." It noted that private American citizens are being strongly urged to depart the country.
Associated Press reporters John Solomon in Washington, Tarek al-Issawi in Dubai, United Arab Emirates and Maggie Michael in Cairo, Egypt contributed to this report.