High-ranking KGB official Vitaly Yurchenko departed Washington for Moscow late yesterday on a special flight of the Soviet national airline, leaving behind an embarrassed Central Intelligence Agency and a series of questions, some of which senior administration officials say may never be answered.

CIA Director William J. Casey has told the White House that Yurchenko's return to the Soviet Union after his apparent defection is a public embarrassment but does not represent a major intelligence failure for the United States, according to informed sources.

Casey, these sources said, maintains that whether Yurchenko was a genuine defector who changed his mind or a double agent, he did not take any valuable CIA secrets back to Moscow.

The president, in an interview, suggested there could be a pattern in the Yurchenko case and two other apparent defection incidents. "You can't rule out the possibility that this might have been a deliberate ploy or maneuver," he said.

New details were revealed yesterday about Yurchenko's final moments in CIA custody. According to Senate sources briefed by the CIA, Yurchenko's last words to his CIA security officer Saturday night at the Georgetown restaurant Au Pied de Cochon were: "If I leave, will you shoot me?"

When the officer said he would not, Yurchenko said, "If I'm not back in 15 minutes, don't blame yourself."

The KGB official did not return. He found his way a mile north to the Soviet compound near Calvert Street and Wisconsin Avenue, somehow persuaded security guards there to admit him and then spent the rest of the weekend preparing for the Monday evening news conference in which he announced he had been kidnaped, drugged and held against his will.

Last night, a Canadian television network reported that authorities were investigating whether the suicide in Toronto of the wife of a Soviet trade official was linked to the mystery woman who was said to be the object of Yurchenko's affections.

["We have found no connection" between Yurchenko and the woman who jumped from the 27th floor of a Toronto building, Canadian spokesman Sean Brady told Washington Post correspondent Herbert H. Denton. Intelligence sources said last night that Yurchenko's former lover is the wife of a Soviet diplomat in Ottawa.]

One senior intelligence source described Yurchenko for the first time as an "enforcer" with high-level administrative and security duties as the No. 2 man in the KGB department supervising intelligence operations in North America. Yurchenko may have known important Soviet secrets, this source said, but may not have been a spymaster who actively directed covert agents and spy rings in the field.

This view was supported last night by a source who has worked with U.S. intelligence agencies, who said he had information that Yurchenko was a colonel working in a department that included numerous generals of the KGB, and that descriptions of him as one of the top five officials in the KGB were ridiculous. "I don't think he was No. 50," this source said.

This assessment of Yurchenko's importance contrasts with earlier ones, leaked to the news media while the CIA was debriefing him, that characterized him as the No. 5 man in the Soviet KGB and his defection as a major coup for the United States.

Yurchenko's 4:15 p.m. departure from Washington Dulles International Airport on the Aeroflot Ilyushin jet, which had an hour earlier brought Soviet Ambassador Anatoliy Dobrynin from Moscow where he had participated in presummit negotiations with Secretary of State George P. Shultz, was called a "happy day" for Yurchenko by a Soviet Embassy spokesman.

"I am looking forward with impatience to returning to my country to see my family, friends and comrades," a statement attributed to Yurchenko said.

Yurchenko carried the traditional bouquet of roses for the departing traveler as he stepped from an embassy van at the Dulles charter terminal to board the flight. In his statement, Yurchenko reiterated his charges that he had been "kidnaped and forcibly detained by the American security services." The statement concluded that he would respond to additional questions from journalists in Moscow.

His departure left the CIA facing intense congressional skepticism over whether the agency had been duped by Yurchenko from the day he walked into the U.S. Embassy in Rome on Aug. 1 or whether the agency so bungled the delicate process of debriefing and psychological support for Yurchenko that it drove away the man the agency has consistently described to Congress as its most important intellgence defector in decades.

One Senate source said yesterday that some members of the Senate Select Committee on Intelligence have asked the CIA to review hundreds of hours of debriefing sessions conducted with Yurchenko to determine if his questioners tipped him off to U.S. intelligence secrets.

Sen. David L. Boren (D-Okla.), a member of the intelligence panel, said "there were significant lapses in what I consider acceptable standards of security and surveillance" in the Yurchenko case.

The CIA, however, has defended itself against such criticism. The agency told the White House, according to informed sources, that it had adopted a deliberate strategy of trying to win Yurchenko's confidence over time, in part by convincing him that even a Soviet defector was allowed rights and freedoms in the United States.

The agency said it may have fallen victim to this strategy, but still thinks it was the right approach, these sources said.

The sources, elaborating on the agency's strategy, pointed out that the CIA changed its approach after it received considerable criticism for its treatment in the late 1960s of Yuri Nosenko, a Soviet defector who was kept in isolation and subjected to abuse over 3 1/2 years.

With Yurchenko "we were going for the long ball," said one senior intelligence official, "and that meant waiting him out, gaining his trust, convincing him that American freedoms were real." The real intelligence prize could have come if Yurchenko had disclosed anything relating to sophisticated overhead satellite systems, elaborate electronic surveillance or about possible moles in the U.S. government, the agency believed.

There were indications yesterday that President Reagan had accepted the CIA's version of the affair. During an interview with news agency reporters, he said, "There's no way to establish" whether a defector is sincere or not.

"You just have to accept that we did our best," Reagan said.

The CIA's approach to Yurchenko was based on the assumption that he was a bona fide defector, though the CIA has acknowledged that he might have been a double agent in a "sting" operation from the first. Several members of the Senate intelligence panel have said there is mounting evidence that Yurchenko was a Soviet plant, including one senator who said he had been told that, on some occasions, Yurchenko evaded questions and complained that the CIA was guarding him too closely.

A Soviet emigre, who has himself been debriefed by the CIA, also supported this view. He said it was extraordinary for the Soviet government to authorize a news conference such as the one that Yurchenko gave at the Soviet compound here Monday, particularly while Secretary of State Shultz was in Moscow to see the Soviet leader and foreign minister.

Such a sensitive propaganda operation must have been approved at a high level in Moscow, the emigre speculated. Noting that only a handful of the most senior Soviet officials have ever been allowed to go before western reporters for spontaneous news conferences, this emigre speculated that Yurchenko was a double agent from the beginning, a man in whom Soviet authorities had complete faith.