More than anything, John Kowalczyk loved his two children, but he was barred from seeing one son and permitted to visit the other only once a week because of a bitter custody dispute with his ex-wife.

"He was a wonderful father to these boys," his fiancee, Lisa Zumwalt, said yesterday in her first interview since the widely publicized slaying of the Fairfax County builder. "All he wanted was to see them and have a relationship with them."

One year ago tonight, Zumwalt was waiting for Kowalczyk, 38, to return from a Vienna parking lot where he was to meet his ex-wife, Katherine, and drop off his 12-year-old son. The builder, sitting at the wheel of his pickup truck, was shot to death.

The case became one of the area's biggest mysteries, a tale colored by a nightmarish custody battle, suicide and an unexplained disappearance. The saga continues, with a murder trial scheduled to start this summer, pending lawsuits and a police investigation covering three states -- Virginia, West Virginia and Florida.

"He was absolutely in love with life," Zumwalt, 40, said of her fiance. "He always had time for the people he loved."

For months, police scrambled for clues, chasing scores of leads before settling upon this conclusion: Kowalczyk's former father-in-law, Stanley Hyman, of McLean, paid to have him killed. Hyman, a retired Air Force lieutenant colonel, wanted to avenge bad blood between Kowalczyk and Hyman's daughter, police said. To do the job, he allegedly hired Ralph Shambaugh Jr., a maintenance worker known as J.R. who worked at a resort Hyman frequented in West Virginia.

Police have telephone records linking Shambaugh and Hyman, according to sources familiar with the investigation. Shambaugh called Hyman three hours after the killing and he deposited about $9,000 in $100 bills in a West Virginia bank within a few weeks of the slaying, the sources said.

Just as police were closing in, however, Hyman and his wife, Jacqueline, who both had denied any involvement in the Kowalczyk slaying, were found dead in a Florida condominium: Police called it a murder-suicide. Shambaugh later was arrested and charged with capital murder, conspiring with Hyman and using a firearm. Shambaugh, 33, is jailed in Fairfax County and faces trial Aug. 23.

There's enough circumstantial evidence and finger-pointing for a juicy television movie -- police have heard from several Hollywood types -- but whether there's enough material to convict Shambaugh is another matter.

Shambaugh "swears he did not do it, that he did not shoot John Kowalczyk," said Peter D. Greenspun, Shambaugh's attorney. "And that he did not conspire with Mr. Hyman to kill John Kowalczyk... . It's a whodunit."

Indeed, many issues remain unresolved:

* Why did Stanley Hyman shoot his wife and then himself last summer, two months after Kowalczyk's death?

* Exactly how was the murder plan hatched? Whose idea was it to shoot Kowalczyk in the head while his son, Nicholas, was nearby, certain to discover the body?

* Where is James Alting, 38, a friend of Shambaugh's who disappeared last June 19, nine days after Kowalczyk was slain?

Police searching Alting's home in Berkeley Springs, W.Va., found a piece of paper with Stanley Hyman's telephone number and a matchbook from the Wolf Trap Motel in Vienna, not far from the parking lot where Kowalczyk was shot. But Alting's role in the case remains unclear, much to the frustration of his parents.

"I don't think he had anything to do with what happened in Vienna, but you never know," said Alting's 66-year-old father, Robert.

Some new details could emerge at Shambaugh's trial. Others could come in a $10 million civil lawsuit filed by Kowalczyk's sister against the Hyman estate and Shambaugh; that case is set for trial in January.

Katherine Kowalczyk, who has moved from the area with her sons, now 13 and 10, could not be located for comment. She previously has declined interviews about the case.

Kowalczyk was killed a few months before he and his ex-wife, who were divorced in October 1992, were to return to court in a custody dispute.

A judge had barred Kowalczyk from seeing his then 9-year-old son, Michael, after a psychologist said the child feared him. Kowalczyk was convinced that his ex-wife and in-laws had turned the boys against him, according to Zumwalt and others.

"I believe the Hymans were doing all they could to ruin John -- financially, personally, and in his relationship with the children," Zumwalt said.

Zumwalt, who does patent and trademark work, met Kowalczyk in 1987 when they were taking a real estate course. She said they began dating in 1991, the same year that Kowalczyk moved out of his home and into an Oakton apartment.

Zumwalt had weathered her own share of marital troubles, having been divorced in 1989 from James G. Zumwalt, the son of Adm. Elmo R. Zumwalt Jr., former chief of naval operations. She had custody of their two children, now 12 and 15.

"Because of the unhappy marriages we had, we never took each other for granted. Never," Zumwalt said in an interview at her Herndon home. "We knew that what we had was really special, and we really treasured it."

They spent a lot of time together, going to the District waterfront to buy fresh fish for barbecues, roller-blading with her two children, going to New York to see Broadway shows and dancing in ballrooms and polka halls. They had planned a wedding for last July 10.

Zumwalt plans to spend today with Kowalczyk's parents and other members of his family. Yesterday, she got flowers from a girlfriend with a card saying: "Thinking about you always. Especially tomorrow."

The pain hasn't eased any in the past year, Zumwalt said, sometimes crying as she talked about her fiance. "We were happy together. We were very much in love. We were looking forward to growing old together."