WASHINGTON (AP) _ Top-ranking KGB officials believe Lee Harvey Oswald was incapable of carrying out the Kennedy assassination alone, ABC News reports in an upcoming program.

The KGB officials based their informal opinion on a wealth of information about Oswald the Soviets had gathered during his years in the Soviet Union, ABC News executive producer Tom Bettag said in an interview Thursday.

ABC interviewed the KGB officials as part of a story on Oswald's KGB file that will be broadcast on ''Nightline'' Friday, the 28th anniversary of President Kennedy's assassination.

ABC does not name the officials who offered the view that Oswald wasn't sophisticated enough to have carried out the assassination by himself.

But the officials were totally familiar with the contents of Oswald's 10- inch-thick KGB file, Bettag added.

ABC spent a day reviewing Oswald's KGB file with assistance from a Soviet translator and a KGB case officer.

It shows that the Soviets suspected Oswald was an American agent when he defected, conducted surveillance of him and bugged his residence, says Bettag.

''This is the most detailed profile you could possibly have and it could only be gathered in a police state,'' said Bettag. The file contains no conclusion about Oswald and the assassination of Kennedy.

But it shows that the KGB inserted informants to work with Oswald at his factory job and that there was electronic eavesdropping of his conversations with his future wife, Marina, said Bettag.

''They were trying to figure out what he was all about,'' said Bettag.

The KGB file shows that the Soviets even allowed Oswald to buy a rifle and monitored his target practice.

The file contains news clippings about Oswald after the Kennedy assassination.

Oswald was an ex-Marine who went to the Soviet Union in September 1959. While there, he married and returned to the United States in June 1962 after obtaining permission from the Soviets and the U.S. State Department.

ABC spent two months in the Soviet Union after the attempted August coup, talking with the new head of the KGB, Vadim Bakatin, and with the KGB case officer in charge of the files. ABC also went to Minsk, where Oswald lived after defecting from the United States.

The network wasn't allowed to make copies of Oswald's KGB file and the Soviets eventually withdrew their cooperation and refused to allow further access to it.

Some conspiracy theorists on the Kennedy assassination have suggested that Oswald was an American agent working for the CIA or the FBI. The agencies have denied the suggestion.