When veteran local prosecutor Peggy Frossard first saw Cathleen Crowell Webb recanting on television three weeks ago a rape accusation that sent a man to prison for six years, she said, her "gut reaction was, 'She's telling the truth.' "

Frossard, 33, an assistant Cook County state's attorney who has prosecuted more than 100 rape cases and won most of them, became deeply involved in a detailed review of the Webb case.

But, in one of the twists that have made the case so bizarre, Frossard's original belief in Webb's new story began to fade as she explored the evidence more thoroughly.

Today, Frossard said, she is convinced that Webb's recantation is a fabrication, and that Gary Dotson raped her as Webb had alleged in 1977.

"Why is Cathy doing this?" Frossard asked. "We just don't know, we can't get inside her mind."

Frossard was aided in reinvestigating the case by J. Scott Arthur, who originally prosecuted Dotson. Both prosecutors today voiced skepticism about reports that Webb took and passed a privately administered polygraph test that supports her recantation.

In Menomonee Falls, Wis., the Associated Press reported, Webb's attorney, John McLario, told reporters that the test contained 10 questions.

[He said Webb was asked whether she was with Dotson at the time of the alleged rape in 1977, took part in a sex act with him, had any physical contact with him or took part in a sex act with anyone on that date. She answered "no" each time, and the examiner found her responses "truthful," McLario said.]

Arthur said polygraph test results are not admissible evidence under Illinois law.

In her recantation, Webb said that she fabricated the charges against Dotson to cover up fears of pregnancy after having sexual relations with a teen-age boyfriend. At the time, she was 16.

Webb's recantation freed Dotson from prison in Joliet for a week, while evidence from his trial was reviewed by the original judge, Richard Samuels.

At the hearing on Dotson's fate last week, medical evidence that Webb had sexual relations with a man was introduced. The prosecution contended that the evidence, which included a semen stain on her underwear and a pubic hair recovered from her body, implicated Dotson.

But Dotson's lawyer, bolstered by Webb's recantation, contended that the weight of evidence did not conclusively link Dotson to Webb's sexual encounter. Samuels rejected the defense argument.

Samuels, a longtime criminal court judge, cited precedent rejecting the validity of recanted testimony and last Thursday ordered Dotson, 28, to continue serving his 25- to 50-year sentence.

Since then, Samuels has been sharply critical of publicity surrounding the hearing, and others have criticized Samuels for refusing to reverse himself.

Dotson's family and friends have launched a petition drive to free him from jail and are seeking a pardon from Illinois Gov. James R. Thompson or another remedy. Dotson is eligible for parole in 1988.

Thompson's office has said Dotson must follow the normal procedure of filing a petition for clemency with the state Prisoner Review Board, which would make a recommendation to the governor. A board official has said a review of Dotson's case could not be completed before the end of the year.

"As rape cases go, this case is overwhelming," Arthur said. "Given her injuries, her demeanor, the kind of [evidence] she gave, who she was, the evidence is that this was a rape."

When Arthur learned that Webb was recanting her accusations, he said, "I was skeptical. I said to myself, 'Hey, this is baloney.' "

Frossard said she thinks that Webb's reversal may have "a chilling effect" on women's willingness to report rapes.

"Studies have shown that people are somewhat reluctant to come forward [after] a sexual attack. Somehow, we must counteract this. I have had some concern on the effect on future rape victims," she said.