FEDERAL agents, admittedly frustrated by their 17-year-old hunt for the elusive killer known as the Unabomber, are intensifying their search by tapping the recollections of thousands of ordinary citizens like David Schusteff. From 1971 to 1976, Mr. Schusteff taught biology in Chicago's northern suburb of Niles, where investigators believe the Unabomber might have attended high school in the 1960s or '70s. Last Friday morning, Schusteff was one of about 50 teachers gathered in an air-conditioned conference room at the Niles North High School as FBI agents displayed a sketch of the Unabomber, described his ideology, and asked the educators to ''jog their memories'' for former students who fit the picture. ''Yes, I had a few recollections of kids I taught in the 1970s who were a little off, a little radical,'' says Schusteff, who spoke privately with the FBI agents. But though he recalled ''a lot of free thinkers and free speakers,'' Schusteff said none of his former pupils closely matched the description of the Unabomber, who since 1978 has killed three Americans and injured 23 others with handmade package bombs. The Federal Bureau of Investigation (FBI) officials suspect that the Unabomber is acting alone, unlike the Oklahoma City or World Trade Center bombings, which allegedly involved people working in groups. That makes him harder to track down, FBI agents say. Investigators are forced to rely heavily on information volunteered by citizens to identify the bomber. ''We are dealing with what we believe to be a single individual who has managed to evade identification for over 17 years,'' says George Grotz, spokesman for the FBI's Unabomber task force based in San Francisco. ''That makes it very difficult to develop any kind of an intelligence base. ''The best we have to go on is the composite sketch and the [public] hot-line information'' Grotz says. Since it was set up in 1993, citizens have called the hot line (1-800-701-2662) with 20,000 leads on the case, including 5,000 this summer alone. To follow up on the leads, the FBI has increased the number of agents working full- time on the case from 50 in 1993 to 80 today. Thirty of the agents are based in Chicago, Sacramento, Calif., and Salt Lake City, with the rest in San Francisco. To encourage a broad flow of tips from the public, the FBI has avoided divulging the details of its profile of the bomber - even as agents in Chicago narrowed their investigation this month to specific high schools. Federal officials suspect that the bomber may come from the Chicago area, where the first three attacks occurred. In 1978, a bomb injured a student in the parking lot of the University of Illinois at Chicago. A year later, a second bomb injured a student at Northwestern University in Evanston, Ill. The third, a letter bomb, exploded in June 1980 at the home of a United Airlines executive in Lake Forest, Ill. Following investigations at the two universities, FBI agents in recent days have approached teachers and school district officials in Niles Township and Chicago's northwestern suburb of Maine Township. At Friday's meeting in Niles, agents briefly discussed the manuscript that the bomber mailed to national publications in late June. ''They asked if any student came to mind who had a philosophy that might be echoed in the manifesto,'' says school district spokesman Jeff Berkwits. ''A number of teachers offered some insights,'' he said. The FBI sent the manuscript to 75 scholars in the US, hoping they can trace the ideas to a former student or colleague. The tract, entitled ''Industrial Society and its Future,'' attacks the industrial revolution and its consequences as ''a disaster for the human race,'' saying that while greatly increasing life expectancy, they have ''destabilized society, have made life unfulfilling, have subjected human beings to indignities, have led to widespread psychological suffering (in the third world to physical suffering as well) and have inflicted severe damage on the natural world. ''Until the industrial system has been thoroughly wrecked,'' the manuscript says, ''the destruction of that system must be the revolutionaries' ONLY goal.'' Some 20 FBI agents in Chicago have also questioned machine-shop owners who might have provided training to the bomber. ''He would have had to receive some training in machining or mechanics from the way he's been able to put together the letter bombs,'' says FBI spokesman Bob Long in Chicago.