BOSTON, NOV. 5 -- The Massachusetts Supreme Court ruled today that lawyers must keep their clients' secrets even after the client dies, and one result is that the shooting death of Carol Stuart and the apparent suicide of her husband, Charles, may never be explained.

The court's 4 to 1 ruling was a blow to prosecutors, who wanted to compel testimony before a grand jury from John T. Dawley, a lawyer who represented Charles Stuart. Officials had hoped Dawley could shed light on a mystery that drew national attention and rocked Boston last winter.

Authorities have said they believe Charles fatally shot his wife, who was seven months pregnant, Oct. 23, 1989, as part of a grisly hoax to collect her life insurance.

Prosecutors said they think Charles also shot himself after shooting Carol, tossed a bag of evidence to a younger brother, then told police that he and Carol had been attacked by a black mugger.

But officials admit they are not certain what happened. Carol died in surgery the night of the shooting, followed 17 days later by her prematurely delivered son. Last Jan. 4, Charles Stuart died when he apparently jumped from the Mystic River bridge into Boston Harbor as police were zeroing in on him as a suspect in the case.

One day earlier, Stuart's brother, Matthew, had gone to Suffolk County District Attorney Newman Flanagan and presented evidence linking Charles to Carol's killing. The day before that, Charles had spoken to Dawley.

Believing that Dawley was one of the last to speak to Charles Stuart, Flanagan asked the state's highest court to compel his testimony before the grand jury, whose probe of Carol's death has bogged down.

The court disagreed, saying, "The mouth of the attorney shall be forever sealed." The court said a lawyer's confidentiality belongs to the client and that only the client can forfeit it.

In response to the ruling, Flanagan issued a terse statement promising to reconvene the grand jury "to hear additional testimony" and declining further comment while the investigation continues.

A lawyer representing Carol Stuart's relatives, the DiMaiti family of suburban Medford, said the DiMaitis were upset by the court's decision because it "impedes the fact-finding process."

The ruling was welcomed, however, by the lawyer for Matthew Stuart, who has not been charged but remains a "target" of the grand jury probe, according to authorities. Attorney Nancy Gertner also stressed that there is no reason to think Dawley's testimony would have been useful. She said Charles Stuart lied to and manipulated so many people before and after the shooting that it is quite likely he was deceiving his lawyer too.

Dawley issued a brief statement saying he was happy that the court "reaffirmed the right of every citizen to repose the utmost confidence in his attorney."