The government has called on law enforcement personnel across the nation to remain vigilant against domestic terrorism until March 11, extending a standing alert through the Winter Olympics in Salt Lake City.

The FBI informed 18,000 law enforcement agencies it was extending the alert that was issued publicly Dec. 3 by Homeland Security chief Tom Ridge, FBI and White House officials said. The new advisory came Wednesday, without a public announcement from the Bush administration.

Homeland Security spokesman Gordon Johndroe said Thursday the new alert was "based on the continuing high level of generalized threat information."

The new March 11 end date extends the alert past the Winter Olympics in Salt Lake City, scheduled for Feb. 8-24.

"History has shown there have been security issues" at previous Olympics, said White House spokesman Ari Fleischer. A bomb exploded at the Olympic games in Atlanta in July 1996, killing one woman and injuring 111 people.

March 11 will also be the six-month anniversary of the terrorist attacks, but several officials said they did not believe that influenced the new ending date.

The Dec. 3 warning was based on intelligence that attacks were being planned, possibly to coincide with Islam's holy month of Ramadan or other religious events. Administration officials said on Dec. 17 they would keep it in effect at least through the holiday season.

Ridge plans to survey the Olympic site and discuss security there on Jan. 10, Johndroe said.

A day before that, Ridge will visit the Nevada Test Site, which may become a counterterrorism facility. That day he also he plans to view a facility at Los Alamos, N.M., that helps cities and law enforcement agencies prepare for emergencies.