Philadelphia ? Sex offenders who have been paroled or placed on probation can be forced to take routine lie detector tests as a condition of their freedom, a federal appeals court ruled.

The 3rd U.S. Circuit Court of Appeals said Tuesday that such mandatory tests are constitutional as long as the subjects can refuse to answer questions that might implicate them in crimes.

The ruling, by a three-judge panel, upholds a growing national practice of using polygraphs to monitor rapists, child molesters and other sex-crime convicts who have completed their prison terms but have been judged by psychologists to be likely to commit another crime.

The 3rd Circuit directly covers only Pennsylvania, Delaware, New Jersey and the Virgin Islands. But the ruling’s influence could be broader. Courts and lawyers in other states can cite it as a precedent.

The case involved Albert M. Lee, 36, a Delaware man who pleaded guilty in 2000 to possessing child pornography and having sex with a 15-year-old girl he met through the Internet.

A judge sentenced Lee to almost four years in prison. Lee also was ordered to submit to frequent polygraph examinations after his scheduled 2004 release.

Lee’s attorney, Christopher S. Koyste, argued that ordering someone to answer questions as part of a polygraph test violates the Fifth Amendment protection against self-incrimination. But Circuit Judge Robert J. Ward disagreed, saying that while Lee had to submit to the polygraph, he was free to refuse to answer specific questions.

Koyste said he was considering whether to appeal.

Courts have banned lie detectors from being used as evidence in most criminal trials. And in October, the National Research Council said lie detectors also should not be used to screen government employees.

The council’s report said a “century of research” has provided little evidence that the tests are accurate.

That finding already has been cited by lawyers challenging routine exams for people on probation and parole. In Colorado, for example, a truck driver convicted of sexual assault filed suit in December seeking to halt that state’s practice of forcing some sex offenders to take a lie detector test every six months.