Federal officials are increasingly concerned that a terrorist attack is being planned for the weeks prior to the Nov. 2 presidential election.

"Every day there is new information that raises the level of anxiety," said one law enforcement official who declined to be identified. The concern, based on intelligence reporting of multiple sources, is that the plots or plot may be in the final stages of planning — and ready to be executed at any time, the official said.

On Sept. 13, Attorney General John Ashcroft (search) held a conference call with U.S. attorneys throughout the nation, according to senior federal law enforcement officials.

During that call, Ashcroft warned them of the possibility of a major terror attack on U.S. soil in upcoming weeks. Law enforcement throughout the country were to be made aware of this concern.

Law enforcement officials are drastically increasing their efforts to track down potential terrorists, and have been ordered to press their sources and conduct interviews with anyone who may be potentially connected to terrorism.

"We're asking law enforcement officers to vigorously find any legitimate reason to arrest people we believe could be involved in terrorist activity, such as go after them for visa violations and deport them," one official said, who described the overall effort as one of "shaking the trees."

Senior officials said they "absolutely believe" someone will attempt a terror attack inside the United States within the next four to six weeks. One official described this time "as a very, very serious period."

"I have not seen people in our intelligence communities this nervous since 9/11," one official said, referring to the Sept. 11, 2001 (search), attacks on the United States that killed almost 3,000 people.

Officials said they believe they "disrupted" at least one major plot by some recent arrests, but fear they may not have "derailed" this plot and possibly two others.

"We will need some luck and the grace of God to prevent this one," one official said, explaining that again they have no specifics as to the exact time and place or clear method. They have some intelligence suggesting a truck bomb or some type of bomb may be used.

U.S. officials said they are getting intelligence from a variety of sources, including some people who've been arrested but are now talking. The information will lead to some visible activity in the next 10 days to two weeks, such as a rounding up of individuals who may be tied to terrorism.

FOX News' Anna Persky and Rita Cosby contributed to this report.