The Nuclear Regulatory Commission has closed public access to its online document library, pending a review to determine what potentially sensitive documents should be removed because they might be useful to terrorists, the agency said yesterday.

"Agency guidelines provide that any information that could be useful, or could reasonably be expected to be useful, to a terrorist in a potential attack should be withheld," the NRC said in a statement.

The action came after a report by NBC that the agency's Web site included detailed information on the location of radioactive substances, generally used in medicine and for industrial purposes, that could be used to prepare a "dirty bomb."

In some cases, the data included diagrams that pinpointed the location of the material in hospitals and other facilities, according to the NBC report.

As part of the review, the NRC said it temporarily closed public access to its online document library, its electronic hearing docket files and NRC staff documents related to NRC consideration of a high-level nuclear waste repository.

"This action, when completed, is intended to ensure that documents which might provide assistance to terrorists will be inaccessible while maintaining public access to information regarding NRC activities," the agency said.

After the terrorist attacks of Sept. 11, 2001, more than 1,000 documents were removed from the NRC Web site. Additional documents disappeared in subsequent reviews.

While the Web site does not contain classified material, the NRC "is widening its review to remove additional information that could potentially be of use to a terrorist," the agency said.