NEW YORK -- Before Sept. 11, you could have visited the Federation of American Scientists' Web site for diagrams and photos of U.S. intelligence facilities. You could have gone to another Web site and learned of gatherings at North Dakota's Minot Air Force Base. And you could have gone online and ordered maps of military installations.
Concerned they could be aiding terrorists, some government and private Web sites have decided to stop sharing quite so much potentially sensitive data.
Such measures would not prevent terrorists from turning to libraries or even other Web sites for information that could be useful in attacks.
"But that is not a justification for publishing it in easily accessible ways. Let them work for it," said Steven Aftergood, senior research analyst at the scientists' group.
The private organization removed from the Web its research containing locations, building layouts and aerial images of intelligence offices, some unacknowledged by the U.S. government. Also removed were details on nuclear sites abroad.
Aftergood said discussions in his organization about what should go online are not new, but before Sept. 11, "the threat was not as tangible as it has unfortunately become."
Minot removed clues about where personnel may be gathering, along with schedules of activities and locations of military housing units.
The National Imagery and Mapping Agency suspended online and offline sales of maps of military installations as well as its highest-resolution maps of other U.S. locations.
The U.S. Office of Pipeline Safety now restricts its mapping software and pipeline data to industry and government officials, while the Environmental Protection Agency removed information on chemical plants and their emergency response plans.
"People have a right to know what kinds of risks there are, but unfortunately terrorists are people, too," said Jim Makris, the EPA's emergency coordinator.
The reports are still available in EPA reading rooms, but Makris said identification is required.
Jeremiah Baumann of the U.S. Public Interest Research Group, a private watchdog organization, criticized the EPA's decision and said the data "would not be very useful to terrorists."
Still, Baumann's group took down a 1999 report based on the EPA data to avoid provoking a debate on public disclosure at this time.
Removing material from the Internet is also occurring for non-security reasons.
Some online forums have deleted hate messages attacking Muslims. A news site, Irish Republican Activist Radio, suspended operations out of fear it may be accused of supporting terrorism and see it assets seized.
The removed or restricted materials represent only a tiny piece of what is available on the Internet, but First Amendment advocates are worried this is only the beginning.
"It's a fine balance that must be struck here, but in wartime, the temptation is always to greater censorship rather than less," said Adam Powell, vice president for technology and programs at the Freedom Forum.
The full extent to which terrorists used the Internet to plot the World Trade Center and Pentagon attacks is unclear. But one hijacker who listed a flight school as his address on a pilot's license may have gotten it from printed brochures or the Internet, while some of the hijackers used online travel sites such as Travelocity to buy their airline tickets.
Attorney General John Ashcroft told Congress that one person in federal custody had downloaded information about crop-dusting planes, which could be used for biological or chemical attacks.
William C. Martel, professor of national security affairs at the Naval War College in Newport, R.I., called the attempt to remove material from the Internet futile.
"We can easily come into the category of shutting down the Internet," he said. "Think of how many mundane pieces of information can be used for ill purposes."
Saying the benefits outweigh the potential for misuse, Trip.com has continued to operate its FlightTracker service, which gives real-time information on the speed, altitude and location of flights in the air.
Boeing Co. reviewed its Internet offerings the day of the attacks and left everything up. Said spokesman Bob Jorgensen, "If it has the potential of aiding and abetting the enemy, it's not out there in the first place."