The rumors had circulated among University of Virginia students who packed the auditoriums for Physics 105 and 106: Some people weren't writing their own papers. Finally, one student took her suspicions to the professor.

A decade ago, it would have been hard for Lou Bloomfield to sniff out plagiarism in a class that draws as many as 500 students a semester. Last month, though, he designed a simple computer program to look for any common phrases and set it loose on his electronic database of 1,500 term papers. His heart sank as his computer churned out one match after another.

"It was a little more common than I hoped," he said.

Bloomfield's search has triggered U-Va.'s biggest cheating investigation in memory, with a total of 122 students facing possible expulsion for cheating. A blow to the university's famed honor code, the staggering number of cases is forcing the student-run disciplinary system to work overtime. Among the accused are students who graduated a year or more ago and some who are scheduled to receive their diplomas in less than two weeks.

Yet some faculty and student leaders take a strange satisfaction in the scandal. For years, they feared that computer technology made it too easy for students to crib from Web sites or to cut-and-paste someone else's essay. In Bloomfield's class, the computers finally bit back.

"Technology really is a double-edged sword when it comes to cheating," said Thomas Hall, student chairman of the university's honor committee. "The means for detecting cheating are catching up with the means for cheating."

Added David T. Gies, a longtime Spanish professor: "It will send a wake-up call to those students who have forgotten what the community of trust is all about."

Plagiarism has become an increasing concern in the age of the Internet. The World Wide Web offers students access to exotic research materials once far out of reach. But many educators fear it offers a powerful temptation as well, to borrow the words of others with a simple click and drag of the mouse.

A survey last year of 2,200 students at 21 colleges found that 10 percent admitted they had borrowed fragments of material they had found on the Internet, while 5 percent said they had taken large passages or entire papers. "It's providing a simpler technology" for those who are inclined to cheat, said Don McCabe, a Rutgers University professor who conducted the study.

Several commercial ventures now offer Web sites or software to help teachers ferret out plagiarism. At the University of California at Berkeley, one professor used such a service to run the work of 340 neurobiology students through an Internet search engine; 45 students were found to have stolen material.

Such programs may have had little effect in Bloomfield's class, though, where students were apparently stealing not from the Internet but from each other.

Bloomfield's wildly popular two-part course, "How Things Work," offers an introduction to the physics of everyday life -- how an airplane flies, how a television works -- taught in laymen's language. Students say he is famous for his lively in-class demonstrations -- spraying a fire extinguisher to propel himself across the room on a skateboard, making a light bulb shine by microwaving it in a cup of water.

This semester he enrolled 320 students, last semester, 520 -- a class so large that students sit in three auditoriums. Bloomfield stands in one room, his lectures broadcast by closed-circuit television to the two others.

Along with exams, students are required to turn in one 1,500-word paper that describes the physics of common technology, such as a helicopter or cell phone. Papers are submitted by e-mail.

The student who brought her complaints to Bloomfield was bitter because she had received a low grade on her paper, the professor said. Meanwhile, she said she knew of many other students who had simply borrowed essays written by friends who had earned A's in previous semesters.

"There are always stories of files being kept of old papers," Bloomfield said, "but I had never heard of it being made real."

He designed his program to scan papers and identify any that shared phrases of at least six words. The computer rarely stumbled upon six-word matches in papers that otherwise appeared to have been written independently. But almost every time it found a six-word match, it found long passages in common, up to cases where "virtually the entire paper is the same."

He now realizes that the medium of e-mail, which made it so much easier for him to collect and grade his papers, may also have enabled students to casually spread and swap their work.

"Technology has made some of the easy ways out very seductive and blurred the lines between what's acceptable and what's not," Bloomfield said. "Cheating is on a gray scale. Things come rolling into your computer, and you feel ownership of them even if you don't own them.

"You slide down the slope into full-fledged intellectual theft."

One student whose work is under investigation -- a 1999 graduate now living and working out of the state who asked not to be identified -- said he is an innocent victim. If his work matched another, it is because he lent his original work to a friend -- a common practice among students, he said.

"People might ask, 'Hey, how did you do that paper?' " simply seeking hints for a useful format, he said. "I was pretty free with it. I assumed under the honor system they wouldn't use it."

In fact, many of the accused will probably be exonerated, honor system officials said, because their paper was the original.

"Only half these people turned in someone else's work," Hall estimated. "The question is how complicit the sources were. That will be the real tough issue."

Hall said honor committee members will treat each case individually, giving priority to those students poised to graduate this month. He predicted it will take until October to handle all the cases. Officials said diplomas will be taken from any students who have since graduated.

He and others said they welcome the debate the case is likely to trigger, saying it will strengthen the honor system.

"One of the things we have not yet understood is the power and potential rascality of the Internet," said Spanish professor Gies, a former faculty senate president. "I don't think we've trained students yet about what is fair and not fair."

Other U-Va. professors are talking about trying similar computer safeguards. That could rattle some at a university where the honor code has traditionally meant less scrutiny, not more.

But even in the most honorable system, Bloomfield argued, you still need some kind of enforcement.

He believes his plagiarism search sent a powerful message. Word got out about the honor investigation a week before this semester's term papers were due. When he tested the latest batch, he found almost no plagiarism.

"It was a very fast educational process," he said.