WASHINGTON – Doubts are mounting here about the authenticity of a letter from Al Qaeda’s deputy chief to the terror organization’s field commander in Iraq.

The letter was made public October 11 by President Bush’s new director of national intelligence, John Negroponte, in a press release that said, “The United States Government has the highest confidence in the letter’s authenticity.” If the letter were to be proven a forgery, it would be a blow to the credibility of an office that was created last year at the urging of the National Commission on Terrorist Attacks Upon the United States following the intelligence failures of September 11, 2001.

Mr. Bush quoted the letter Saturday during his weekly radio address. “We intercepted this letter, and we have released it to the public,” the president said of the July 9 letter from Ayman al-Zawahiri to Abu-Musab Zarqawi.

The letter includes criticism of Mr. Zarqawi’s tactics of targeting Shiite civilians in the war in Iraq. It also includes a warning that Iran would retaliate for the targeted suicide bombings aimed at Shiite communities, stressing that angering Iraq’s neighbor was poor strategy, given that the government there is still holding 100 of Al Qaeda’s operatives.

The CIA and the State Department have said the 6,000-word correspondence provides a unique window into the thinking of the enemy in the war on terror. But over the weekend, Al Qaeda in Iraq issued a statement from Mr. Zarqawi claiming the letter was a fake. “Everything in the letter attributed to Ayman al-Zawahiri is false,” the statement said. Some analysts inside and outside the administration now are also taking this line.

“This does not read like an Islamist text,” a terrorism analyst at the conservative-leaning Hudson Institute, Chris Brown, said in an interview yesterday. “It only uses the word ‘infidel’ twice and makes five references to ‘crusaders.’ They are talking about the U.S. military in Iraq, which all Islamists, including Al Qaeda, always refer to as the crusader nations, but in this letter Zawahiri refers to America almost exclusively.” Mr. Brown added that the letter also uses references to both the Christian and Muslim calendar: “We know that Al Qaeda leaders will use the Islamic calendar in private correspondence and the Christian calendar for statements meant to be public, but never both in an internal communication.”

Mr. Brown also notes that the letter, which was dated two days after the London underground bombings, makes no mention of the attacks carried out by Al Qaeda’s minions in Britain. “It’s quite amazing that they went to great lengths to assure that Al Qaeda would get credit for the subway bombings and they would make no mention of it in this letter, especially considering that Zarqawi is also commander of all operations in Europe,” he said.

Mr. Bush’s chief of staff, Andrew Card, appeared at a Hudson Institute event earlier this month.

Two sources inside the Bush administration told The New York Sun yesterday that some lower-level analysts share some of the concerns Mr. Brown raised, but that at the highest levels of the government there is no questioning the letter’s authenticity. “Some of us think there is a possibility that a foreign intelligence service may have faked it,” one administration source said.

When the letter was released last week, the chairman of the House Permanent Select Committee on Intelligence, Rep. Peter Hoekstra, warned against drawing too many conclusions from single-sourced intelligence. The statement from Mr. Hoekstra, a Republican of Michigan, said: “There will be significant discussion over the coming days and weeks as to its exact nature and intent. The letter may offer possible insight into the current state of Al Qaeda, but I caution against reading too much into a single source of intelligence.”

Other observers, however, say they have reason to believe the letter is real. One expert on Al Qaeda, Peter Bergen of the New America Foundation, yesterday cited Mr. al-Zawahiri’s preoccupation with winning over Muslims as a reason why he suspected the July 9 letter was real. “One of Zawahiri’s preoccupations is, ‘We don’t have the masses on our side.’ In his book, ‘Knights Under the Prophet’s Banner,’ he says two causes the masses can get behind are Palestine and the United States’ intervention. These themes are in the letter. That is consistent with his thinking,” Mr. Bergen said yesterday. “I don’t have sufficient confidence in our intelligence agencies to think they would be able to dream up a fake like this.”

The Washington editor of al-Hayat, Salameh Nematt, said he thought the letter was written by Mr. al-Zawahiri, but it was not meant as a private correspondence but as an extended press release. “They are not supposed to be creating a rift between fellow Muslims. Al Qaeda would have to issue a statement to that effect,” Mr. Nematt said. “This was not meant to be sent privately to Zarqawi, this was meant to clear their names publicly. In a sense they are disassociating themselves from killing innocent Muslims. Whether this was intercepted, I don’t know. My guess is that this was meant for some kind of media or Al-Jazeera.” When asked about the recent statement from Mr. Zarqawi, Mr. Nematt pointed out that no such denial has yet been issued by the letter’s alleged author. “Zawahiri has Web sites, too,” he said.