Robert Maheu, who was a powerful aide to reclusive tycoon Howard Hughes and whose cloak-and-dagger exploits included involvement in a CIA and Mafia plot to assassinate Fidel Castro, died Monday at Desert Springs Hospital in Las Vegas. He was 90 and had cancer and heart ailments.

Maheu was a one-time FBI agent who ran a Washington company that he said carried out secret missions for the Central Intelligence Agency.

Maheu’s first jobs for Hughes in the 1950s included private-eye snooping on Hughes’ past and prospective girlfriends in Hollywood. Later, as Hughes’ chief adviser, he helped make his boss Nevada’s third-largest landowner, after the federal government and the state power company.

After becoming Hughes’ director of Las Vegas operations in 1966, Maheu was the most influential member of the billionaire’s inner circle and acted as his liaison to leading political figures and the world at large.

“If he wanted someone fired, I did the firing,” Maheu wrote in his 1992 autobiography, “Next to Hughes.” “If he wanted something negotiated, I did the bargaining. If he had to be somewhere, I appeared in his place. I was his eyes, his ears, and his mouthpiece.”

Before he was abruptly fired in 1970, Maheu spoke with Hughes as many as 20 times a day on the telephone. But in all their years together, he never met the eccentric mogul face to face. Hughes lived in seclusion on the top floor of the Desert Inn Hotel, with only a few private aides admitted to his presence.

“He finally told me that he did not want me to see him because of the way in which he had allowed himself to deteriorate, the way in which he was living, the way he looked,” Maheu said on “Larry King Live” in 1992. “He felt that if I ever in fact saw him, I would never be able to represent him.”

Maheu earned $520,000 a year and was living in one of the largest houses in Las Vegas when Hughes had two other aides fire him in December 1970. In 1972, Hughes broke a long silence by speaking in a telephone news conference, seeking to prove he had nothing to do with a purported autobiography by Clifford Irving, which was later confirmed a hoax.

During that news conference, Hughes called Maheu “a no-good son of a bitch who robbed me blind.” Maheu sued him in federal court for defamation. He initially won a $2.8 million settlement from Hughes, but the decision was overturned.

The four-month trial revealed many engrossing details about Hughes’ business dealings, his political contributions and his increasingly bizarre private life.

Maheu disclosed that in 1970 he delivered $100,000 to Charles “Bebe” Rebozo, a close friend of President Richard Nixon’s, in return for possible future favors for Hughes. Maheu entertained Nixon’s vice president, Spiro Agnew, on his yacht and regularly played tennis with then-Nevada Republican Gov. Paul Laxalt, who became a U.S. senator.

But Hughes spread his political largess to both parties, contributing $100,000 to 1968 Democratic presidential candidate Hubert Humphrey.

Maheu said he personally placed a briefcase containing $50,000 cash — from receipts at the Hughes-owned Silver Slipper casino — in Humphrey’s limousine. The contributions were legal at the time because they were considered private donations from an individual, not corporate contributions.

Maheu said he twice turned down requests from Hughes to arrange $1 million payments to Presidents Lyndon Johnson and Nixon — payable after they left office — if they would agree to stop underground nuclear testing in Nevada, where Hughes lived until moving to the Bahamas in 1970. (He died at age 70 in 1976.)