Fairfax County Prosecutor Robert F. Horan Jr. said his hands were tied.

He said he found it "morally reprehensible" that four or five teen-agers would watch as a 15-year-old Fort Hunt High School student was raped in a school bathroom. But he said he could take no legal steps against the youths.

"There is nothing to charge them with," Horan said Wednesday, when he announced that two other youths were being charged with rape in the Oct. 3 incident in which rape is alleged. "If the public wants that changed, the place to do that is the legislature, not the commonwealth's attorney's office," said Horan. Another man, Jeffrey Lewis, 19, was charged last month with abduction with intent to defile in the same case.

"I feel angry," Susan G. Wibker, a sexual assault therapist at the Fairfax County Victim Assistance Network, said of the onlookers' escape from legal liability. "It's a statement to the victim that it's nothing to make a big deal out of, it's not that important."

The legal situation in Virginia is not unique. Under the law in Maryland, the District and all but three states, bystanders have no legal duty to step in to stop a crime -- or even to pick up the phone to alert police.

"In our system of law, we're not our brother's keepers . . . ," said George Washington University law professor Gerald M. Caplan. "The common law has a cold, heartless quality to it."

Law professors' favorite classroom example is that of the Olympic swimmer who sees a baby drowning in a shallow pool of water. Under the traditional American rule, derived from centuries of English law, the swimmer has no legal responsibility to rescue the child.

"Conscripting people to do good runs against very strong traditions in the American legal system," said New York University Law School Professor David A.J. Richards, who favors a change in the law. "The law ends up really reinforcing the worst impulses of citizens."

Most European countries take a different approach. In West Germany, for example, a bystander can be sentenced to up to a year in prison for failure to render aid in an emergency situation where "he would not subject himself to any considerable danger" or violate another legal duty.

"Just about every Western country has a duty-to-assist law," said Harold Takooshian, a Fordham University psychologist who has studied the problem of bystanders' inaction and organized a three-day conference earlier this year on the 20th anniversary of Kitty Genovese's murder. Genovese was stabbed to death outside her New York apartment building as 38 of her neighbors ignored her cries for help.

Until recently in this country, only Vermont had such a duty-to-assist law, imposing a $100 fine for failing "to reasonably assist" a person in grave danger.

Last year, Rhode Island and Minnesota enacted similar laws, spurred by reports of the rape at Big Dan's tavern in New Bedford, Mass., where four men raped and sodomized a woman on the bar's pool table as the bartender and four patrons looked on. Two of those patrons were later charged with aggravated rape because prosecutors said they cheered the attackers on, but they were acquitted.

In Rhode Island, witnesses to a rape who fail to notify police immediately face fines of up to $500 and imprisonment for as long as one year.

The Minnesota law provides a $100 penalty for failure to render "reasonable assistance" to anyone who is "exposed to or has suffered grave physical harm." It also allows victims of the crime to file civil damage suits against witnesses who failed to come to their aid.

"It doesn't mean you have to jeopardize your own life, but at a minimum you must call the police," said state Rep. Randy Staten, who sponsored the Minnesota measure.

None of the three laws has yet been enforced, according to Takooshian. But, he said, they serve an important moral function simply by being on the books. "Once something becomes a law, that strengthens the moral statement," he said.

Not everyone favors such statutes, however. "It creates the danger of people who take desperate acts because they're afraid they'll be in trouble with the law," said Robert Kane, a prosecutor in the Big Dan's case. "To be a coward is not to be a criminal," he added. "There are some weak and timid people out there."

Whatever the legal situation, some social scientists suggest that the problem of bystander inaction is particularly prevalent in crimes such as rape.

"When there's a man attacking or fighting with a woman, what bystanders generally do is make an assumption that the two people know each other," said R. Lance Shotland of Pennsylvania State University. "Once having decided that, they tend to say he's not really hurting her, she doesn't really want help."