An unidentified associate of Sinatra offered the legendary crooner's help in tracking down "reds" in Hollywood in 1950, according to internal agency memos.

At the time, Sinatra was apparently upset over press reports that painted him as a communist sympathizer.

The offer to cooperate with the feds as a red-hunting informant was just one of dozens of startling revelations contained in 1,275 pages of secret FBI files released yesterday.

Among the allegations:

*Sinatra was the target of three criminal probes, including alleged death threats made in 1971 to a computer-stock salesman.

Federal investigators believe Sinatra and New York crime kingpin Carlo Gambino invested thousands of dollars in the stock, which quickly tanked.

The man who set up the deal was threatened with death if he didn't return the $100,000 put up by Gambino, Sinatra and another man.

*He was investigated in 1955 for possible perjury involving a sworn affidavit in which he denied having been a communist or member of the Communist Party.

*Probers ordered another perjury investigation into Sinatra in 1960 after he denied having Chicago mob boss Sam Giancana as a guest at a wild, celebrity-studded party that lasted two weeks at an Atlantic City hotel.

*Throughout Sinatra's career, mobsters secretly held part ownership of his entertainment empire, and had "a piece" of several other major entertainers, including Dean Martin, Dinah Shore and Tony Bennett.

*Sinatra acted as a respectable "front" for mobsters looking to invest in restaurants, hotels and casinos.

*He and presidential candidate John F. Kennedy took part in wild sex orgies in hotels around the country. Sinatra often went to extraordinary lengths to keep JFK's activities hush-hush.

*The FBI spoke to several prostitutes who had "professional visits" with Sinatra, and his name appeared on a client list for a suspected "white slavery" ring.

Despite the avalanche of accusations and coast-to-coast investigations, Sinatra was never charged with any mob-related crimes.

The mountain of information, innuendo and underworld snitching about the legendary crooner dates back to 1946, when a dour J. Edgar Hoover scrawled his first note about Sinatra on an internal memo.

After reading a news article about frenzied teen-age girls waiting for Sinatra at a concert, Hoover wrote:

"Sinatra is as much to blame as are the moronic bobby-soxers."

For the next three decades, the feds kept close tabs on the singer, largely because they were trying to build racketeering cases against the mob chieftains who partied with him.

The FBI was especially interested in his relationship with Sam Giancana.

"Anything that Sinatra does, Giancana is part of it," one mob informant told the feds. "Sinatra enjoys surrounding himself with hoodlums and believes that [he] would give up his show-business prominence to be a hoodlum if he had the courage to do so," another informant is quoted in the papers.

But many of the informants are contradicted by other informants, and in one case just dead wrong.

Back in 1971, a snitch told the FBI Sinatra had cancer and had no more than two months to live.

The report sparked a flurry of high-level memos around FBI headquarters.

Sinatra lived for another 27 years. He died last summer at the age of 82.

In a bizarre twist, one of the officials most interested in Sinatra's mob ties was Nixon aide John Ehrlichmann.

Ehrlichmann requested and received a classified account of the FBI's suspicions - including allegations that the singer's parties served as cover for mob sit-downs.

Soon after his request, Ehrlichmann was the one under investigation, and eventually went to prison for his role in the Watergate coverup.

The Sinatra file was released following a Freedom of Information request from various news agencies.

Sinatra received the same pages in 1980 after filing a personal request.

Linda Kloss, who oversaw the release, said that only 25 pages of the 1,300-page file on Sinatra were still being kept secret.

-- Devlin Barrett - 12/10/98