LOS ANGELES -- Dramatically illustrating the security problems posed by the rapid growth of the Internet computer network, one of the United States' three nuclear-weapons laboratories has confirmed that computer hackers were using its computers to store and distribute hardcore pornography.


Embarrassed officials at Lawrence Livermore National Laboratory in Livermore, California, which conducts a great deal of classified research and has highly sophisticated security procedures, said the incident was among the most serious breaches of computer security ever at the lab east of San Francisco.


The offending computer, which was shut down after a reporter investigating Internet hacking alerted lab officials, contained more than 1,000 pornographic images. It was believed to be the largest cache of illegal hardcore pornography ever found on a computer network.


While hackers once devoted their efforts to disrupting computer systems at large organizations or stealing electronic information, they have now developed ways of seizing control of Internet-linked computers and using them to store and distribute pornography, stolen computer software, and other illicit information.


The Internet, a "network of networks" originally designed to connect computers at universities and government research labs, has grown dramatically in size and technical sophistication in recent years. It is now used by many businesses and individual computer users, and is often viewed as the prototype for the "information superhighway" of the future.


But the Internet has an underside, where so-called "pirates" with codenames like "Mr. Smut," "Acidflux," and "The Cowboy" traffic in illegal or illegally obtained electronic information. The structure of the Internet means that such pirates can carry out their crimes from almost anywhere in the world, and tracing them is nearly impossible.


The FBI late last week confirmed that it was investigating software piracy on the Internet. A reporter discovered a number of sites at prestigious institutions that were being used to distribute stolen software, including one in the office of the president of the University of California, Berkeley, and another at Lawrence Berkeley.


At Lawrence Livermore, officials said Monday that they believed at least one lab employee was involved in the pornography ring, along with an undetermined number of outside collaborators. Chuck Cole, deputy associate director of computing at the lab, said that nearly 2,000 megabytes of unauthorized graphical images have been found in a Livermore computer, and he confirmed that they were pornographic.


The employee has been placed on "investigatory leave" and his or her security badge confiscated while an investigation is undertaken, the lab said. It was unclear whether the pornographic images were being sold or how many people had gained access to them. The pictures were sufficiently graphic that they would be likely to be considered obscene by the courts, and therefore transmitting them over the Internet would be illegal.


The massive amount of storage capacity used in the Livermore scheme shows how Internet hacking could be quite profitable. Seizing control of large and sophisticated computer systems at universities or government labs can save unscrupulous entrepreneurs large sums of money.


There were indications that the person operating the pornography database had become aware of possible scrutiny. On June 27, a message left in a file labeled "Read Me!!!" said, "It appears that news about this site has escaped. In the past two weeks, I have had 27 unauthorized hosts attempt to access my server. This does not give me a warm-fuzzy feeling. I would hate to have to shut this down, but I may have no choice."


One computer expert suggested that the hardcore pornography may be a cover for an ultrasophisticated espionage program, in which a "sniffer" program combs through other Livermore computers, encodes the passwords and accounts it finds, and then hides them within the pornographic images, perhaps to be downloaded later by foreign agents.


But Cole said there was no possibility of a computer intruder gaining access to classified data at Livermore labs.


The problem of pirate sites extends far beyond U.S. government labs and universities: Many popular sites are located in Mexico, France, Britain and other foreign countries.


The Software Publishers Association, a trade association representing major software manufacturers, has made software piracy on the Internet a major priority. Peter Beruk, the association's litigation manager, said: "We are currently tracking over 1,600 pirate sites on the Internet in a joint investigation with the FBI. It is a very serious and costly problem."