WASHINGTON — As an American-born spokesman for Al Qaeda threatens to blow up American embassies abroad, intelligence gleaned from last month’s British “doctors plot” of car bombers suggests that a Qaeda cell is on the loose in the American homeland.

E-mail addresses for American individuals were found on the same password-protected e-mail chains used by the United Kingdom plotters to communicate with Qaeda handlers in Europe, a counterterrorism official told The New York Sun yesterday. The American and German intelligence community now believe the secure e-mail chains used in the United Kingdom plot have provided a window into an operational Qaeda network in several countries.

“Because of the London and Glasgow plot, we now know communications have been made from Al Qaeda to operatives in the United States,” the counterterrorism official said on condition of anonymity. “This plot helps to connect a lot of stuff. We have seen money moving a lot through hawala networks and other illicit finance as well.” But this source was careful to say that at this point no specific information, such as names, targets or a timeline, was known about any particular plot on American soil. The e-mail addresses that are linked to Americans were pseudonyms

Since the thwarted plot in London and Glasgow over the weekend of July 1, American intelligence officials have gone public with their concerns about a Qaeda presence in America. The secretary of homeland security, Michael Chertoff, said he had a “gut feeling” on an increased risk of attack. More recently, the commander for Northern Command, the American military operational region that includes North America, Air Force General Victor Renuart told the Associated Press in an interview on July 25, “I believe there are cells in the United States, or at least people who aspire to create cells in the United States.” He added, “To assume that there are not those cells is naive and so we have to take that threat seriously.”

The heightened threat environment also played a role in the passage of a law Saturday evening to give the National Security Agency temporary authority for the next six months to conduct warrantless wiretaps against suspected al Qaeda contacts on American soil. On Friday, as Congress was still wrangling over the specifics of the changes to the Foreign Intelligence Surveillance Act, the director of national intelligence, Admiral Michael McConnell, was unusually blunt about the threat. Commenting on the Democratic proposal that the White House rejected, Admiral McConnell said, “I must have certainty in order to protect the nation from attacks that are being planned today to inflict mass casualties on the United States.”

While al Qaeda in the past has threatened attacks on American soil, the latest public threat from the terror organization vowed specifically to attack American embassies and interests overseas, such as its 1998 attacks on the American embassies in Kenya and Tanzania.

In an Internet video released over the weekend, a bomber known as Abu Othman pledged to attack embassies, which he said are used to spread moral degradation and encourage women to display themselves and wear make up.

In the same video, a Qaeda member and adviser on American culture, Adam Gadahn, also known as Azzam the American, said, “We shall continue to target you, at home and abroad, just as you target us, at home and abroad, and these spy dens and military command and control centers from which you plotted your aggression against Afghanistan and Iraq.”

Mr. Gadahn, who has been on the FBI’s most wanted list since 2004, was born and raised in rural California and was a devotee of death-metal music before joining Al Qaeda, according to a profile of him earlier this year in the New Yorker.

Al Qaeda in June released a video from a training camp in Pakistan featuring graduating terrorists who pledged to conduct attacks on British soil and in other Western countries.

In light of the concern over possible sleepers in America, the counterterrorism official yesterday said he thought the video may be a feint. “We are more concerned about sleepers here than we are about hardening any more our embassy targets,” the official said.

The British press has detailed Qaeda connections to the London and Glasgow plots. One of the ringleaders, an Iraqi doctor named Bilal Abdulla, was reported to have been recruited by Abu Musab Zarqawi in 2005 for attacks abroad. The declassified version of the National Intelligence Estimate says that Al Qaeda in Iraq has developed an operational capability to hit targets outside of Iraq.

The British press and the New York Times reported that two of the British plotters tried at one point to enter America using visas for medical workers.