The FBI already had been investigating reports that the government's KH11 spy satellite system had been compromised when it stumbled across William Kampiles, a federal court jury was told yesterday.

Testifying at Kampiles' espionage trial here, FBI counterintelligence expert Donald E. Stukey acknowledged under cross-examination that the bureau had "other suspects" in mind before the 23-year-old Kampiles, a former CIA watch officer, suddenly came to official attention.

Defense attorneys carried the questioning no further. U.S. District Court Judge Phil M. McNagny had ordered them a secret hearing yesterday morning to ask how the government learned the KH11's secrets might have been lost to the Soviets.

The judge told reporters during a recess that he did not regard that line of questioning as relevant. He also said that "national security" was a factor in his ruling.

It appeared, however, that the early FBI investigation, and the secrecy attached to it, might have some connection with the KH11's capabilities and Soviet efforts to thwart them.

According to testimony at the trial, now in its second week, the sophisticated satellite system reportedly carried an ultrasecret classification known as "UMBRA."

If it was so labeled, sources say, the KH11 might be able to perform electronic eavesdropping - on Soviet long-distance telephone calls, for example - as well as photographic reconnaissance.

When first interviewed by FBI and CIA officials Aug. 14 at the Twin Bridges Marriott in suburban Virginia, Kampiles told them that the KH11 manual he remembered seeing at the CIA had some pages marked "Top Secret UMBRA."

Government prosecutors, however, have done their best to play down that recollection and have suggested that Kampiles may have been mistaken. The only copies of the KH11 manual introduced in evidence at the trial are marked "Top Secret ROUGH."

In testimony earlier this week, the CIA's deputy director for science and technology, Leslie Dirks, explained the distinction. The code-word ROUGH, he said, is a special security designation for photographic reconnaissance work. The UMBRA designation is reserved for "non-photographic" intelligence information, "specifically communications intelligence." Dirks said.

One source said that an UMBRA designation would mean that the KH11 can not only photograph Soviet missile sites from 100 miles up, but can also gather electronic intelligence for the National Security Agency, primarily by picking up microwave signals and relaying them back to earthbound monitoring stations.