A Fairfax County jury yesterday sentenced Ralph N. Shambaugh Jr. to 18 years in prison for conspiring to kill builder John Kowalczyk, but a mistrial was declared on the capital murder and gun charges against the West Virginia groundskeeper.

Jurors spent nearly 20 hours in deliberations before telling Judge Richard Jamborsky that they still were deadlocked on whether Shambaugh had killed Kowalczyk last year in a Vienna parking lot. The panel convicted him Tuesday of conspiring with McLean millionaire Stanley Hyman to kill the 38-year-old Kowalczyk, Hyman's former son-in-law.

The jury, which could have given Shambaugh a 20-year term, also fined him $25,000 -- an amount they chose because it was what the prosecution said Hyman paid for the slaying. Shambaugh's attorney said he could be eligible for parole in about four years.

"We would have liked to have seen a little more evidence. ... You're always looking for a smoking gun, and that was certainly lacking," juror Philip S. Mershon, a Fairfax City council member, said later in the courthouse lobby. "The majority believed he {Shambaugh} was, in fact, the triggerman."

After the six-man, six-woman jury delivered its verdict, Commonwealth's Attorney Robert F. Horan Jr. said he would attempt to retry Shambaugh on the murder-for-hire charge, which is punishable by the death penalty. Defense attorney Peter D. Greenspun said he will research whether his client can be tried a second time in the case.

If the 34-year-old Shambaugh is retried, Jamborsky said, it may be necessary to move the trial out of Fairfax because of the publicity. The issue likely will be discussed at a hearing next month.

Although Hyman was implicated in the conspiracy, he was not on trial in Courtroom 5-E. Hyman killed himself and his wife, Jacqueline, in a Florida condominium two months after Kowalczyk's June 10, 1993, slaying and a month before he was named in the indictment.

Before yesterday's sentencing, Horan called the murder plot "the worst form of conspiracy one can engage in ... a conspiracy to remove another human being from the face of the earth." He told jurors that Shambaugh was convicted in 1979 of unarmed robbery, but he was not allowed to mention that Shambaugh has a malicious-wounding charge pending against him in West Virginia.

Greenspun countered that though his client "is clearly a country bumpkin," he was also "the non-instigator" in the crime. He called to the stand three West Virginia neighbors who said "J.R.," as he is known, was "very caring" and would cut their grass or shovel their snow free of charge.

Throughout the 2 1/2-week trial -- the longest criminal proceeding in Fairfax history -- Horan maintained that the conspiracy grew out of a bitter custody fight between Kowalczyk and his ex-wife, Katherine Hyman, over their two young sons. Kowalczyk was shot to death in his pickup truck as he was returning his 12-year-old son to Katherine Hyman after a visit.

Although the Vienna police chief testified that he saw Shambaugh in the parking lot shortly after the slaying, no one witnessed the shooting and police never found the murder weapon. A West Virginia man testified that Shambaugh told him he was offered $25,000 to kill a Fairfax man but later said he had changed his mind and was returning the money.

When jurors reported yesterday that they were unable to reach a verdict on the murder-for-hire charge, Horan asked the judge to tell them that they could find Shambaugh guilty if they believed he at least was an accessory to the killing. Jamborsky declined to issue that instruction, saying jurors might think the judge was telling them how to vote.

Juror Timothy J. MicKinney Jr. said last night he was "probably the strongest advocate for acquittal" among the four panelists who voted against a murder conviction because "I felt the state produced absolutely no evidence" that Shambaugh actually pulled the trigger.

"I believe he did enter into a contract with Stanley Hyman, or possibly other family members," said McKinney, a 48-year-old lawyer, "but I couldn't leap from that conspiracy charge to murder. There was a wall for me."

Horan was able to prove a conspiracy using phone records that showed more than 30 calls between Hyman and Shambaugh in the six weeks before Kowalczyk's death -- and none after. "If it weren't for the phone records," Mershon said, "I don't think the case would have supported conspiracy."

The jurors included two lawyers, two teachers, a Federal Express Corp. employee and two defense consultants.

The jury's first vote on the capital murder charge was 7 to 5 for conviction, Mershon said; only one juror crossed over for a guilty verdict at the end.