Fighting back tears, I. Grace Viscidi pleaded yesterday for the return of her 12-year-old son Billy, who has been missing from his home in suburban Vienna since July 25.

"I want my boy," she told reporters gathered at the Viscidis' house. "If Billy is able to come home, we want him home."

While Mrs. Viscidi was making her appeal, Vienna police said yesterday they are questioning an 18-year-old man, in custody in Arlington, in connection with the boy's disappearance.

Craig L. Bevil, 18, who was charged Friday with sodomy and molestation in a separate case involving a 10-year-old Arlington boy, is considered a suspect, Sgt. Joe Tavares of the Vienna police said.

According to Tavares, who heads the police investigation, Bevil had helped search for Billy in woods near the Viscidis' home and had visited "several times" with the Viscidi family during the search. Bevil could remember having seen Billy in the Vienna area about a year ago while he was delivering some fence, Tavares added.

Bevil, who lives at 4605 N. 24th St. in Arlington, was arrested by Arlington police when they accompanied Vienna police to his home to ask him why he had shown so much interest in the disappearance of Billy Viscidi, police said.

During the questioning, according to police, a 10-year-old boy crawled out from under a bed and ran for the door. The boy, whom police described as "really scared," said Bevil had been committing sexual acts with him when police rang the doorbell.

Bevil is being held on $2,000 bond, Arlington police said.

Tavares said Vienna police have administered three polygraph tests to Bevil at the Arlington County jail. All three show that Bevil is not connected with Viscidi's disappearance, Tavares said, but police still consider him a suspect.

In yesterday's appeal, carried by evening TV newscasts, Mrs. Viscidi said: "Billy, I'd like you to come home if you can. We love you and this is your home. We want you back. If anybody out there knows anything at all, if they would please contact us or the police, we want Billy back."

Billy was last seen by his family shortly after he finished his breakfast on Tuesday, July 25. He walked away from his home at 503 Princeton Ter., his mother said, and headed toward nearby Interstate 66. When the boy failed to show up for dinner that night his mother called police.

The 12-year-old's disappearance, which has received widespread media attention for the last two weeks, has taken a toll on the family, his mother said.

When he heard about his son's disappearance, Burton Viscidi returned home early from Georgetown Hospital where he had undergone surgery for the removal of a kidney stone.

Once home, Viscidi, who is a broadcast technician with the voice of America, took part in searches for his son in the thick woods that surround the family's home. He talked to police talked to the press and worried, his wife said yesterday.

"He just did too much," she said. "The whole operation now has to be redone."

Viscidi will undergo minor surgery tomorrow, according to his wife. Major surgery has to be postponed, she said, because her husband is not strong enough "mentally" or physically."

Taking a paid two-week leave of absence from her job as a computer programmer with the Central Intelligence Agency, Mrs. Viscidi said she has spent her time worrying, too, and taking care of her three other sons.

"I've got to go back to work next Monday unless something major happens to my husband," she said.

Bevil, who police say does not look like the composite drawing of a man reported to have been seen with a boy fitting Billy's description, is scheduled to appear on Aug. 21 in Arlington's Juvenile and Domestic Relations Court in connection with the sodomy and molestation charges.

Tavares, of Vienna police, yesterday called the Viscidi case the "most frustrating" he has ever worked on. "You get tired of running down leads that don't turn up anything," he said.

"The worst part of it," Tavares said, "is having to go and talk with the mother, having to tell her you haven't found anything yet."

A $5,000 reward offered by the Vienna Jaycees has failed to produce any substantial leads, Tavares said.