Final Chapter in Murder Mystery? : Slayings: Two detectives conclude that, as officials had earlier maintained, onetime intelligence operative killed his wife and children, then self. Victims’ relative disagrees.

For three years, the most persistent murder mystery in San Diego County has been: Who killed a mysterious Englishman named Ian Spiro, his wife and their three children?

From the beginning, the evidence strongly suggested that Spiro, caught in the grip of financial problems, killed his family before taking his own life.

But Spiro’s background as an intelligence operative on the fringes of Middle East espionage and intrigue gave rise to theories about assassins and terrorists and murderous factionalists bringing their fury to the ritzy enclave of Rancho Santa Fe. The theories were fueled by the facts that no murder weapon was found and that Spiro and his wife seemed to be a happy, loving couple enjoying the good life.

So relentless has been the clamor, particularly from the English tabloid press, that nefarious international elements were behind the killings that Sheriff Bill Kolender brought two veteran homicide detectives out of retirement for a second look.

Today those detectives, after months of sifting evidence, will hold a press conference to lay out their conclusion that the case is just as it appeared from the start: that Spiro, 46, shot to death his wife Gail, 41, and their children Sara, 16, Adam, 14, and Dina, 11, in their beds and then went to the desert and committed suicide by swallowing cyanide.

“The blood evidence, the physical evidence, it all points to one person [Spiro] as the murderer,” said a source close to the case. “And all these other [theories about] the Mossad, the Iranians, the CIA, it just ain’t there.”

In the days before the killings, police said, Spiro borrowed a .357-magnum from a business partner and bought cyanide from a jeweler.

The Sheriff’s Department probe--conducted by retired Detectives Tim Carroll and Ed Stevens--is not expected to put an end to conspiracy theories about the killings.

Already, Ian Spiro’s brother-in-law has blasted the idea that Spiro was a killer. He believes that the murder has its origins in Spiro’s experiences in the Middle East, possibly in his knowledge of the torture and murder of William Buckley, CIA station chief in Beirut, by Shiite Muslim extremists in 1984.

“I can’t see Ian having done this,” said Greg Quarton, an advertising salesman from Vancouver and brother of Gail Spiro. “The only tension between Ian and Gail was over Ian’s work. It was extremely hard on Gail’s nerves.”

Ian Spiro, a commodities broker, owed several million dollars to creditors because of failed business dealings, including a venture to export Porsches from London to Japan. But Quarton says those problems did not appear to be insurmountable “and were nothing Ian could not have overcome.”

Still, investigators found indications in Gail Spiro’s diaries that the couple were experiencing several marital problems and that she had considered leaving her husband, a source said.

In his book “Terry Waite & Ollie North: The Untold Story of the Kidnapping and the Release,” British journalist Gavin Hewitt asserts that Spiro knew both Waite and North and had made contacts with Shiite Muslims in an effort to gain the release of Western hostages.

Quarton said that he recently found among Spiro’s possessions a business card from a man he identified as a kingpin among Middle East arms dealers.

To the disappointment of Quarton, the sheriff’s investigation apparently centered on blood, fingerprints, ballistics tests and autopsy results, and did not attempt to unravel the tangled web of Spiro’s international business and intelligence dealings.

At the time of the slayings, the Spiro family had only recently moved into a $5,000-a-month rental home in Rancho Santa Fe, with the children enrolling in private schools and Gail Spiro moving swiftly into the social whirl.

She and the three children were found shot Nov. 5, 1992. Spiro initially could not be located, but his body was discovered three days later in his Ford Explorer in a remote canyon at Anza-Borrego Desert State Park east of San Diego.