ALBANY – Detaining sexual predators in mental facilities after they are released from prison could have a serious impact on plea bargaining in sex crime cases, legal observers said.

Last month, both houses of the Legislature passed bills that would allow the state to lock away convicted sex criminals in mental facilities after they complete their prison sentences. The convicted criminals would be sentenced to civil confinement if a jury in a separate trial concludes that they are sexual predators.

The confinement of sexual predators could alter the dynamic of plea bargaining by sharply reducing the incentive for defendants to enter into agreements with prosecutors to plead guilty in exchange for lighter sentences or lesser charges.

The potential that the accused sex offenders could be candidates for civil confinement may lead them to risk jury trials rather than admit to a crime that could result in a lifetime commitment to a mental ward, legal observers said.

“I think it’s very powerful speculation that this kind of bill would take away most of the incentive for anyone to plea guilty because they would be facing the prospect for indefinite confinement potentially for the rest of their lives,” a New York University law professor who specializes in criminal procedure, Stephen Schulhofer, said.

“It would force defendants to go to trial,” Assemblyman Joseph Lentol, a Democrat who represents a Brooklyn district, said. “It could very well bog down the court system with trials. That’s something we need to address in our bills.”

Mr. Lentol is co-chairman of the joint conference committee the Assembly and the Senate are holding to bridge their differences between the two civil confinement bills. The primary difference between the two bills is that the Assembly version gives jurors a choice of sentencing a sexual predator to civil confinement or to “strict and intensive” supervision.

There is little empirical evidence showing a connection between civil confinement and plea bargain rates. One recent high-profile case in Kansas, however, shows how civil confinement has given prosecutors a safety net in cases in which they believe they are better off avoiding a jury trial.

After the Kansas Supreme Court reversed several sex crime convictions against the defendant, Michael Crane, prosecutors decided to avoid the risk of a retrial and negotiated a plea deal with Crane, who served four years for aggravated sexual battery against a female video store clerk. After Crane was released from prison, he faced a civil confinement trial and was sentenced to a mental facility indefinitely.

Proponents of civil confinement say such a preventive statute is needed in New York precisely because sexual offenders are not receiving tough enough prison sentences, a symptom of plea bargaining. Yesterday, a Westchester Assemblyman released data from his office showing that the overwhelming majority of sex crime cases in the county are settled by plea bargains. In 2004 in Westchester, of the 115 people who were charged with a felony sexual offense, 93 pled guilty and 22 went to trial, said Assemblyman Richard Brodsky, a Democrat who is opposing Westchester’s district attorney, Jeanine Pirro, in the state attorney general’s race. Just over a quarter of those who faced a jury trial were convicted, according to the data.

Mr. Brodsky said the figures call out for tighter restrictions on sexual predator plea bargaining and said he strongly disagrees with the notion that civil confinement would have any impact on the decision-making of defendants in sex crime cases. “A rational person would make the judgment,” he said. “Eighty percent of these guys are sociopaths.”

A professor of law at William Mitchell College of Law, Eric Janus, who has written articles critical of civil confinement, said the possibility of being committed to a mental facility is not immaterial for defendants because very few sexual predators are ever released from institutions in states that have passed confinement bills.

“It makes the process of plea bargaining more problematic,” Mr. Janus said. “Civil confinement is basically a life sentence in many states.”

New York would be the 17th state to adopt civil confinement for sexual predators.