Qaeda Plot Leak Has Undermined U.S. Intelligence WASHINGTON - As the nation's spy agencies assess the fallout from disclosures about their surveillance programs, some government analysts and senior officials have made a startling finding: the impact of a leaked terrorist plot by Al Qaeda in August has caused more immediate damage to American counterterrorism efforts than the thousands of classified documents disclosed by Edward Snowden, the former National Security Agency contractor. Since news reports in early August revealed that the United States intercepted messages between Ayman al-Zawahri, who succeeded Osama bin Laden as the head of Al Qaeda, and Nasser al-Wuhayshi, the head of the Yemen-based Al Qaeda in the Arabian Peninsula, discussing an imminent terrorist attack, analysts have detected a sharp drop in the terrorists" use of a major communications channel that the authorities were monitoring. Since August, senior American officials have been scrambling to find new ways to surveil the electronic messages and conversations of Al Qaeda's leaders and operatives. "The switches weren't turned off, but there has been a real decrease in quality" of communications, said one United States official, who like others quoted spoke on the condition of anonymity to discuss intelligence programs. The drop in message traffic after the communication intercepts contrasts with what analysts describe as a far more muted impact on counterterrorism efforts from the disclosures by Mr. Snowden of the broad capabilities of N.S.A. surveillance programs. Instead of terrorists moving away from electronic communications after those disclosures, analysts have detected terrorists mainly talking about the information that Mr. Snowden has disclosed. Senior American officials say that Mr. Snowden's disclosures have had a broader impact on national security in general, including counterterrorism efforts. This includes fears that Russia and China now have more technical details about the N.S.A. surveillance programs. Diplomatic ties have also been damaged, and among the results was the decision by Brazil's president, Dilma Rousseff, to postpone a state visit to the United States in protest over revelations that the agency spied on her, her top aides and Brazil's largest company, the oil giant Petrobras. The communication intercepts between Mr. Zawahri and Mr. Wuhayshi revealed what American intelligence officials and lawmakers have described as one of the most serious plots against American and other Western interests since the attacks on Sept. It prompted the closing of 19 United States Embassies and consulates for a week, when the authorities ultimately concluded that the plot focused on the embassy in Yemen. McClatchy Newspapers first reported on the conversations between Mr. Zawahri and Mr. Wuhayshi on Aug. 4. Two days before that, The New York Times agreed to withhold the identities of the Qaeda leaders after senior American intelligence officials said the information could jeopardize their operations. After the government became aware of the McClatchy article, it dropped its objections to The Times's publishing the same information, and the newspaper did so on Aug. 5. In recent months, senior administration officials - including the director of national intelligence, James Clapper Jr. - have drawn attention to the damage that Mr. Snowden's revelations have done, though most have been addressing the impact on national security more broadly, not just the effect on counterterrorism. "We have seen, in response to the Snowden leaks, Al Qaeda and affiliated groups seeking to change their tactics, looking to see what they can learn from what is in the press and seek to change how they communicate to avoid detection," Matthew Olsen, the director of the National Counterterrorism Center, told a security conference in Aspen, Colo., in July. American counterterrorism officials say they believe the disclosure about the Qaeda plot has had a significant impact because it was a specific event that signaled to terrorists that a main communication network that the group's leaders were using was being monitored. The sharpest decline in messaging has been among the Qaeda operatives in Yemen, officials said. The disclosures from Mr. Snowden have not had such specificity about terrorist communications networks that the government is monitoring, they said. "It was something that was immediate, direct and involved specific people on specific communications about specific events," one senior American official said of the exchange between the Qaeda leaders. The Snowden stuff is layered and layered, and it will take a lot of time to understand it. There wasn't a sudden drop-off from it. A lot of these guys think that they are not impacted by it, and it is difficult stuff for them to understand.