A former Marine Corps officer who wrote a 26-step "recipe for murder" was convicted yesterday of giving life to his fantasies by shooting his wife to death, then hiding her body in the abandoned coal mines of eastern Pennsylvania.

A federal jury in Alexandria took 16 hours to find Robert P. Russell guilty of murdering Shirley Gibbs Russell, 29, a Marine captain who was last seen March 4, 1989, outside the Quantico residential quarters the couple shared before she demanded a divorce.

"I guess I'm the only one who really knows what happened," Russell said before the verdict, adding that he someday will write a book exposing what he said was a corrupt government investigation.

The jurors, who declined to comment, apparently struggled with their decision, announcing yesterday at lunch time that they were deadlocked and later asking U.S. District Judge James C. Cacheris the definition of reasonable doubt. By 6 p.m., the stalemate was broken, but only after Cacheris instructed the jurors to take another look. Russell faces a mandatory life prison sentence.

Ignoring defense claims that Shirley Russell is still alive and in hiding, Assistant U.S. Attorney Lawrence J. Leiser said Robert Russell, 33, "came close" to pulling off the perfect crime. "He's a very bright guy. He thought it through very carefully," Leiser said.

Shirley Russell's body has not been found, no blood was discovered in the Quantico storeroom where Russell is alleged to have shot his wife, and the .25-caliber pistol Russell bought days before the slaying has not been recovered.

But what investigators did find, Leiser reminded the jury during his closing arguments Wednesday, was Russell's own "recipe for murder" -- notes he stored on a computer disk several months before his wife disappeared.

Robert Russell's mother testified during the eight-day trial that the computer entry was the beginning of a novel that she and her son were writing about the murder of a Marine officer's wife. She said she had written "about two chapters" but had thrown them in the trash long ago.

Leiser offered a different view.

"It says what it says," Leiser said, pointing to a five-foot chart duplicating the 26 phrases taken from Russell's computer diary. "Is there any doubt in your mind that this man was planning to murder his wife?"

"Make it look as if she left," Leiser read, noting that Russell told investigators after his wife's disappearance that he last saw her walking to a store five miles away in 40-degree weather to buy a can of paint.

"How do I kill her????" Leiser said, reading No. 15 on the list and highlighting the fact that it was written in the first person, "not as if Jake in the novel was asking himself how he would kill someone."

Defense attorneys Frederick J. Fanelli and Drewry B. Hutcheson Jr. accused the government of character assassination for harping on Robert Russell's many affairs and capitalizing on his drinking problem, which led to a "less-than-honorable" discharge from the Marines in 1988.

The defense lawyers, who announced that they would appeal the verdict, also argued that their evidence suggested that Shirley Russell is still alive.

A handful of defense witnesses testified that they had seen or talked with Shirley Russell hours or, in one instance, weeks after her disappearance.

Darnisha Thompson, a civilian employee at Quantico who said she knew Russell, testified that she saw the captain at Springfield Mall three to five weeks after she failed to turn up for duty and was reported missing.

Thompson said she saw the woman on the other side of the concourse: "She looked at me in the eyes, and it looked as if she knew. She knew I had seen her." Thompson added that the woman, who appeared "disarrayed," vanished before she and her husband could catch up to her.

Three other witnesses reported sightings. One, a clerk at Russell's Quantico apartment complex, said he saw her use a pay phone in the building about 4 p.m. the day she disappeared. A cashier at a Pennsylvania convenience store said Russell came into her store to buy a lottery ticket that same day about 6 p.m., and a Dumfries woman said she saw Russell two days later walking through the snow alongside one of Quantico's main streets.

Hutcheson said the sightings supported the theory that Shirley Russell had grown weary of her demanding military life and the broken relationship with Robert Russell and had chosen to escape it all by going underground. "Shirley's decision was to drop out, to step out of society, to get out of Dodge," Hutcheson said.

Sandy Flynt, a key prosecution witness, said she was not surprised when she heard Shirley Russell had disappeared. Flynt, a teacher at the Stafford County high school where Robert Russell worked in early 1989, said that she and Russell had been having an affair for months when he announced that he was going to kill his wife.

"They had had an argument the night before, and he told me, 'Don't be surprised six months down the road after we get divorced to find out that Shirley is dead. I'm going to kill her,' " Flynt testified.

Flynt said she tried to persuade Russell that the idea was crazy because he would be an obvious suspect. Flynt said Russell had an answer: " 'They may suspect me, they may think I did it, but they won't be able to prove it.' "