Beginning this summer, the Environmental Protection Agency had planned to put information on the Internet about the worst possible chemical plant accident that might occur in specific neighborhoods. Residents and local governments then could ask the facilities near them what safety precautions were in place.

But in August, Congress voted to keep that information off the World Wide Web for at least a year, which says as much about the power of the Internet as it does about chemical safety.

Siding with the chemical industry and federal law enforcement, Congress said the postings would create a national, searchable database for terrorists. "The data would provide terrorists with information on locations around the country where the greatest damage to human health and the environment would occur," said Ivan K. Fong, a deputy associate attorney general whose office oversees information and privacy issues.

Even as Congress blocked the release, however, some of the information already was--and is--on the EPA's Web site.

The Internet was in its infancy in 1990 when Congress amended the Clean Air Act to require thousands of facilities to disclose what could happen in a "worst-case scenario," or a 10-minute release of the largest container of their most dangerous chemical. The information also was to include how far dangerous concentrations could travel and how many people could be affected. Those reports would be in addition to the lists of chemicals that facilities separately file with local governments and already were publicly available.

The new reports were to be sent to the EPA by June 21 and then were to go on the Internet. EPA officials initially discounted the terrorism argument, saying the data set for release did not include crucial information a terrorist would need--where chemicals are stored, what would cause a toxic release and what security measures were in place. Further, EPA officials said, well-marked storage tanks have been around for years without being hit.

"Nobody is going to be sitting in Baghdad like the Wizard of Oz turning dials and saying, 'Where is the best target in America?' " Jim Makris, the EPA's national director of chemical emergency preparedness, said at an international safety conference last year. "The FBI . . . said environmental terrorism is likely to be in this country from a local thug who already knows the chemicals are there or from a disgruntled employee."

But by last fall, the EPA had changed its stance. "As we learned more . . . we began to believe that the national security threats were valid," said Timothy Fields Jr., the EPA's acting assistant administrator for emergency response. Neither the EPA nor law enforcement officials elaborated on those threats.

Critics, led by environmental activists, said keeping the worst-case scenarios off the Internet makes the program useless and shields the chemical industry.

"How many terrorist incidents have we had compared to the number of accidents? None. It's a ludicrous argument," said Paul Orum, head of the Working Group on Community Right to Know, a collection of 1,500 groups.

A spokesman for the Arlington-based Chemical Manufacturers Association said the industry has always supported public disclosure of worst-case scenarios but deferred to the security agencies. "Our concern is how you make it public," said the spokesman, Jeffrey Van.

During the next year, the EPA is to draw up regulations governing public access to worst-case data. Until then, the scenarios are exempt from disclosure under the federal Freedom of Information Act. State and local officials may request the information, but they face fines of up to $1 million if they share that information with residents.

With the worst-case sections removed, the remainder of each report went on the EPA's Internet site, including executive summaries that often contain details of the worst cases, the very information Congress sought to control. A Washington group that advocates public access to government information also has posted about 14,000 such summaries on its Web site, including some worst cases.

The Blue Plains Wastewater Treatment Plant in Southwest Washington, for example, refused a reporter's request for its worst-case information, citing terrorism concerns. But the case--the release of a 90-ton tank car of sulfur dioxide that could send out a toxic cloud with a radius of 15 miles--was on the Internet in the plant's executive summary.

The amended Clean Air Act also requires facilities to hold public meetings to share their safety plans. Six chemical companies in Baltimore recently held an open house and volunteered information about worst cases in brochures.

"It's the right thing to do because we are members of this community as well," said Enrique Bertran, a manager at FMC Corp., in Baltimore, and chairman of the Maryland Chemical Industry Council. Bertran said terrorism concerns originated with the security agencies, portraying Maryland companies as "ready to rock-and-roll" with full disclosure.

"No one is advocating that we keep the worst-case scenario information locked up or away from those communities near to chemical facilities," said Rep. Thomas J. Bliley Jr. (R-Va.), who led the House effort against release. "But we also must ensure that the way this information is provided does not end up harming the very people that Congress intended to protect."

Rep. Henry A. Waxman (D-Calif.) said that if the terrorism argument seems persuasive, "you could come up with this conclusion: Let's don't let anybody have information. . . . The more sensible approach would be to reduce the threat rather than to reduce the flow of information."

Specialists who follow Internet issues said such fights will continue as the nation sorts the implications of making information available worldwide. The Internet makes information "more accessible and promotes great efficiency," said Lance Hoffman, a George Washington University professor who studies cyberterrorism. "But the flip side is it will cause us to reexamine our public records laws."