The acquittal Thursday of a Maryland professor on charges that he sexually exploited his daughter for nearly 20 years illustrates what lawyers say is the difficulty in prosecuting incest cases, especially when alleged victims wait years to come forward.

The professor, Frank J. Munno, 54, head of the University of Maryland's nuclear engineering program at College Park, was acquitted by jurors who said they were unwilling to convict him solely on the word of his daughter, Angela Mattson, 29. Munno has steadfastly denied his daughter's allegations, and some jurors said they needed more evidence before they could find him guilty beyond a "reasonable doubt."

"I knew this case was definitely going to be an uphill battle," said the prosecutor, Beverly Woodard, who obtained a grand jury indictment of Munno in June. Six months earlier, Mattson had come to Prince George's County authorities with her story: Her father, she said, had routinely engaged her in oral sex and intercourse from 1969, when she was 8, until 1987, shortly before she left her family's home in College Park at age 26.

"In a case like this one, the decision {to prosecute} can't be based on the potential of winning," said Woodard, chief of the state's attorney's sex crimes unit. "You know it's going to be a hard road. It's based on whether you believe in your victim. I absolutely believe in Angie."

Yet Munno's three-day trial in Upper Marlboro -- ending with four hours of jury deliberations leading to Thursday's verdict -- offered examples of what lawyers and mental health professionals have described as the often fatal weaknesses inherent in many incest and child sexual abuse prosecutions.

In the Munno case, Mattson wanted jurors to believe that she endured 18 years of sexual exploitation, from third grade through graduate school, without once forcefully resisting her father or seeking help.

Defense attorney Karen Silver, who contended that Mattson fabricated the allegations to get back at her father because of a money dispute, described her to the jury as a physically strong woman trained in martial arts. "She certainly would have had the ability to resist," Silver said in her opening argument. Later in the trial, she placed in evidence a photograph of Mattson as a teenager smashing a board with a karate kick.

Mattson's response, from the witness stand, was that she had been a psychological captive of her father, that she had feared "losing his love," and that Munno had used her fear to manipulate her emotions. Some jurors said after the trial that they found the explanation hard to believe.

Specialists in child abuse and sexual exploitation cases, while stressing they were not discussing Mattson's allegations, said the jurors' reactions were not surprising.

"Jurors always had trouble in my cases with delayed disclosures," said Huntsville, Ala., lawyer Bruce Gardner, a former prosecutor of child sexual abuse cases. "It's very, very difficult for people to fathom that an abusive situation would go on for years and years and be suppressed by the victim. That's the biggest hurdle to get over. Nobody can believe it."

But many jurors, he said, are not aware of a phenomenon that psychologists call "the child sexual abuse accommodation syndrome."

"Incest, one of the most horribly psychiatric-intrusive things that can happen to anyone, bonds the offender and victim in this crazy miasma of guilt and fear," said New York lawyer Andrew Vachss, who devotes much of his practice to representing the interests of young victims of sex crimes.

Fred Berlin, a psychiatrist and director of the Sexual Disorders Clinic at Johns Hopkins Hospital in Baltimore, added: "I don't think anyone has good demographics on what percentage {of victims} respond in that way, but certainly the ambivalence that people feel -- the love-hate relationship -- those are all very common, human reactions in incest victims."

Making the Munno jury aware of such a phenomenon would have meant summoning an expert psychiatric witness, Woodard said. Even if Judge G.R. Hovey Johnson would have allowed such testimony, Woodard and others said, the possibility of its backfiring on the prosecution would have been great.

Although the expert would have explained the phenomenon generally, without referring to Mattson, Silver likely would have argued that the testimony at least had implied that Mattson suffered from "the child sexual abuse accommodation syndrome." Lawyers said Silver could have argued for the right to try to impeach the testimony.

Mattson has been undergoing psychological therapy in recent years, and Silver might have moved to place the therapist's records in evidence, lawyers said. She might also have sought an evaluation of Mattson by a psychologist of the defense's choosing.

"If you bring out a certain psychological dynamic -- this delayed reporting -- then that opens the door for the defense to call in all kinds of experts to try to discredit the victim with her own psychological past," said Gardner, the Huntsville lawyer, who helped write a 500-page manual on courtroom strategy for the National Center for the Prosecution of Child Abuse.

Still another danger: "We've learned the hard way that unless the prosecutor and expert are extremely well prepared and very, very careful, the evidence may be presented in such a way that it oversteps the proper lines for expert testimony," said Patricia Toth, director of the National Center for the Prosecution of Child Abuse. "It can mean the reversal of a conviction on appeal."

As a result, Woodard said, she chose to let Mattson explain herself in her own words, and hoped the jurors would "use their common sense."

Mattson, a nuclear engineer like her father, said she left her parent's home and entered therapy in 1987, shortly after meeting Army Maj. Philip Mattson, to whom she is now married. She testified that she decided to file a criminal complaint against Munno after months of counseling, hoping to inspire others to come forward with reports of incest.

But Silver warned the jury: "Keep your minds open . . . . There is no direct evidence in this case. There will be no physical evidence."

She was correct. Prosecutors often must enter such trials with no physical evidence to support their cases, Woodard and others said.

When an alleged victim -- for example, a sexually active adult -- complains of sexual child abuse years after it supposedly occurred, there is no likelihood of finding, say, genital scars to corroborate the charges, lawyers said. Even when the alleged victim is a child who was recently violated, the lawyers said, physical signs often are difficult to detect.

As for eyewitnesses, there were none in the Munno case, and rarely do any appear in similar trials, lawyers said. "Sex acts do not occur where people can see them," Woodard said in court. "They are private acts."

One of Mattson's two brothers testified to once seeing his father and sister "tongue-kissing," but said he witnessed nothing more.

Mattson's other brother told the jury he never saw any unusual behavior involving his sister and father. Numerous friends, neighbors, relatives and colleagues of Munno's testified to spending many evenings in his home, and described a normal relationship between Munno and his daughter.

Silver told the jurors Thursday, "If there was ever a case with a reasonable doubt, this is it."

Jurors said later that they agreed.