Ever since a hijacked jetliner crashed into the Pentagon on Sept. 11, 2001, parents whose children attend day care there have been assured their kids were safe. But last week, Defense Department officials told them the center would close in the fall because they could no longer ensure the children's safety.

Parents say the sudden pronouncement contradicts the many assurances they have received that the facility -- located in the Pentagon's north parking lot -- is among the safest in the nation. Now the parents are questioning the validity of the government's decision to close it down.

"The parents just aren't buying the security issue," said Jeffrey Altdorfer, whose wife, Laurie, is a civilian employee at the Pentagon and whose 2-year-old son, Jordan, has been enrolled in the Pentagon Child Development Center since December. "It doesn't stack up."

The facility, which once had a waiting list of 100 children, was shut down briefly after the 2001 attack. Though it reopened a month later, officials said it never again reached its capacity of 140 children. The parents of the 117 children who use it say the inconvenience will be widely felt.

Officials have assured parents they will help them find day care, placing many children at the Fort Myer Military Community Child Development Center, about a mile away. But Army officials confirmed yesterday that Fort Myer will be able to accommodate only about 50 additional children when the Pentagon center closes.

Washington Headquarters Services officials said they would join with the Department of the Army to build an expanded day-care center at Fort Myer to serve Pentagon and Fort Myer children. The new center could open as soon as 2007.

But Altdorfer is skeptical of the rationale for closing one facility in favor of building another.

"If it was a concern about location," he asked, "why did they reopen day care" at the Pentagon after Sept. 11? "And if they're really concerned, why reopen it [at Fort Myer] at all?"

Officials began assessing whether it was safe to house child-care centers in or near high-profile government targets, such as federal buildings, after the 1995 Oklahoma City bombing. Several children from an on-site day-care center were among the 169 people killed when a truck bomb destroyed the Murrah Federal Building there.

A study determined that there should not be day-care centers at such sites, but the Pentagon's child-care center remained. Its security was enhanced after the Sept. 11 attacks. Officials said the decision to close was based on subsequent terrorist attacks around the world and worries that the potential for a terrorist attack at the Pentagon could rise with the approach of the national elections.

"The threat assessments have determined it would be imprudent to continue," said Glenn Flood, a spokesman for the Defense Department. "There's no specific threat, but we continue to assess what's out there."

But Jill Wood, who has two daughters at the Pentagon center, wondered how dangerous it really is. "If security is indeed the issue here, they should have briefed us six months ago," she said. "There is no interim plan for us. The people who made this decision won't have to deal with the fallout. We will."

In a memo to top Defense Department officials, Howard G. Becker, the director of Washington Headquarters Services, cited the report that followed the Oklahoma City bombing and concluded that the Pentagon was not a safe place for child care. "One can not avoid the fact that the Pentagon is among the 'highest profile' symbolic buildings in the United States and continues to be a high-interest target of terrorists," Becker wrote. "We conducted a careful review of the structural engineering of the current child care building and an assessment of the potential collateral effects of a second attack on the Pentagon to the children who may be in the center or on its playground at the time and we concluded that with the availability of other day care centers it was advisable to take this course of action at this time."

Lori Collins spent 11 months on a waiting list before securing a place for her daughter at the Pentagon. She is unconvinced that the closure is necessary. "They have put hundreds of thousands of dollars' worth of protection and security measures into the Pentagon over the last few years," Collins said. "This facility is more secure than that at Fort Myer. I don't believe the threat is any more or less than at any other government facility around here."

Flood disagreed, saying there's a difference between housing children near a building such as the Pentagon and keeping them on a sprawling military base, such as Fort Myer.

Flood said care is being taken to make sure parents find space for their children at Fort Myer, at other nearby military child-care centers or at other centers run by Knowledge Learning Corp., the contractor for the Pentagon Child Development Center.

"The bottom line is we're not rushing into closing this thing," Flood said. "Even though we've said the fall, that's an outside goal. We'll be flexible. We aren't going to do anything fast. We're going to do it right."

U.S. Rep. James P. Moran Jr. (D-Va.) said his staff has received several calls from Pentagon parents. "The ultimate decision to move the facility is probably something the parents would have agreed with," Moran said. "But they deserved more consideration. Parents should have been involved at an earlier stage in the decision-making process."

Laurie Altdorfer reads with her son, Jordan, in their home in Dumfries. He attends day care at the Pentagon center scheduled to close in the fall.Like other Pentagon workers, Laurie Altdorfer said she is distressed that her son, Jordan, will lose his day-care center there. The center serves 117 children and has a capacity of 140.