WASHINGTON — One of the world’s foremost authorities on Al Qaeda says that last month’s compromise of the intelligence community’s penetration of the terrorist group’s Internet communication system was a serious blow, but that, ultimately, the damage was not fatal.

The head of the International Center for Political Violence and Terrorism Research at Singapore’s Nanyang Technological University, Rohan Gunaratna, said in an interview yesterday that the damage done on September 7, when ABC News published online quotes from a transcript of Osama bin Laden’s first speech in three years, was “reparable.” But he also called it a “serious breach.”

“This has happened from time to time,” Mr. Gunaratna said. “Each time they suspect penetration of their password-protected sites and other communications, they take security measures to minimize exposure, but they always come back. Each time, however, they are more cautious.”

As The New York Sun reported Tuesday, the September 7 leak tipped off Al Qaeda’s internal security division to a breach in their Web-based “intranet,” a network that American intelligence analysts call “Obelisk.”

Rita Katz, the founder of a private firm, known as the SITE Institute, that tracks Al Qaeda online, provided the video to the National Counterterrorism Center after consulting the White House general counsel, Fred Fielding. Within an hour of the ABC News story, a series of Web sites that Al Qaeda had used for internal communications, processing propaganda videos, and keeping track of clerical matters all went dark to the American intelligence community.

On Tuesday, the office of the Director of National Intelligence said there was no formal investigation into the leak, though inquiries were being made as to how the media had obtained the transcript.

Mr. Gunaratna yesterday referred to the network as the “password-protected sites.” They differ from public Al Qaeda sites used for recruitment and propaganda because the network of these “intranet” domains changes constantly and is used only for internal messaging, not for disseminating information to the public. However, Obelisk sites often exist within public Al Qaeda sites.

Mr. Gunaratna said that Al Qaeda set up its Internet communications in three tiers. The smallest circle includes senior leaders such as the head of the group’s information committee, Abu Abdel Rahman al Magrebi, a son-in-law of Al Qaeda no. 2, Ayman al Zawahari. Mr. Gunaratna estimated that this circle consisted of no more than 20 people, and was nearly impossible to penetrate — although, he said, America, Britain, and Pakistan had a successful operation that penetrated Al Qaeda’s most secretive Web communications for a period in 2004. Then, after one of the founders of Al Qaeda’s Internet system, Mohammed Naeem Nur Khan, had been arrested in secret by Pakistani intelligence in July 2004, allied intelligence services were able to monitor the communications of this leadership group for about five weeks though Mr. Khan’s messages to the top tier committee. The operation was blown after leaks to the Washington Post and New York Times, Mr. Gunaratna said, a breach he considers comparable to the leaking of Mr. bin Laden’s speech.

The next tier of Al Qaeda’s Internet communications consists of the password-protected sites, also known as Obelisk, and is used mainly by middle and lower-level Qaeda operatives.

“We refer to these as the password-protected sites,” Mr. Gunaratna said. “They are time-bound, they will work on this front only for certain people, they change Web sites constantly. You have to be plugged in, it’s like a game, you have to hunt them.”

In yesterday’s interview, Mr. Gunaratna praised Ms. Katz, who had told the Sun that her operation was ruined as a result of last month’s leak to ABC News and other news organizations. Ms. Katz’s work was unique, according to Mr. Gunaratna, because he said Ms. Katz is “obsessive.”

“There are endless methods of deception,” he said. “It is an infinite game, I can deceive you once, but I need to find another way. We have not seen anyone on the government side with the same dedication and passion as Rita in this,” he said.

In another interview yesterday, the ranking member of the House Permanent Select Committee on Intelligence, Pete Hoekstra, a Republican from Michigan, also said he was not surprised at all that Ms. Katz was able to obtain Mr. bin Laden’s video before professional intelligence agencies.

“The Internet and the Web is an area where we have been woefully weak,” Mr. Hoekstra said. “Except for some folks who did this on their own and the Department of Defense, the direction from the top has been woefully weak.”

Mr. Hoekstra said he asked the intelligence community for a translation of the Osama bin Laden speech the day it first appeared on the media. But he did not receive it until the next day, and even then the translation was marked “for official use only.”

“I couldn’t believe it,” Mr. Hoekstra said. “That means I can’t talk about it with reporters like you. So I had my staff go online and get a translation.”