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 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 organization is believed to be headed by Abdulaziz Issa Abdul-Mohsin al-Moqrin, the top al-Qaida figure in Saudi Arabia.

The Saudi official said there had been no communications from the kidnappers except for a video and written statement they 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 kidnappers did not specify what time the countdown began or when it ends. Saudi newspapers on Thursday quoted unidentified government officials as saying the kingdom would not give in to the demands of terrorists.

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.

If Johnson is harmed, it read, “I will never forgive you. I will curse you in all my prayers.” 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, also was 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 had directly communicated with Johnson’s son in New Jersey to apprise him of the full extent of the search.

Dorothy Pharo, left, is consoled by Iris Emas during a vigil for Paul Johnson Jr. at the fire department in Eagleswood Township, N.J. Johnson, who grew up in the town, was taken hostage Saturday in Saudi Arabia. Johnson's captors threatened Tuesday to kill Johnson in three days unless Saudi authorities release al-Qaida prisoners.