In a report published Saturday, a senior official was quoted in the Post as saying that "everything seems to lean toward a domestic source. Nothing seems to fit with an overseas terrorist type operation."

Authorities are considering many possibilities, the Post reports. They include associates of right-wing hate groups and people in the U.S. sympathetic to Islamic extremist causes.

Officials say none of the 60 to 80 threat reports gathered every day by U.S. intelligence agencies has connected the letters containing anthrax spores to al Qaeda or other terrorist groups. They say evidence from the spore samples provides no links to a foreign government or lab.

One senior official told the Post that while authorities believe al Qaeda members are planning more serious attacks, "nobody believes the anthrax scare we are going through is" the next wave of terrorism.

"There is no intelligence on it and it does not fit any (al Qaeda) pattern," the official is quoted as saying.

Only one clue appears to point to foreign terrorist involvement, an official said. FBI behavioral scientists have concluded that whoever wrote the three letters delivered to Senate Majority Leader Tom Daschle, D-South Dakota, NBC News and the New York Post didn't learn English as a first language.

But several officials said the letter-writer could have lived in the United States for a while. And, official of the Simon Wiesenthal Center told the Post that the anti-Israel comments in the letters and the statements of suspected terrorist Osama bin Laden are supported and mouthed by U.S. extremist groups.