Prosecutors often decline to pursue cases against federal prison guards and other employees who have sex with inmates, and when they win convictions the punishment often is light, the Justice Department's internal watchdog reported Monday.

It is illegal for a federal prison employee to have sex with an inmate, even if it's consensual. But fewer than the half the 163 such cases substantiated by investigators and turned over to federal prosecutors resulted in criminal cases against prison employees in the five years that ended in September, Justice Inspector General Glenn A. Fine (search) said.

The major reason is that federal law considers most cases of sexual relations just a misdemeanor with a maximum sentence of a year. It's a felony only if the prison employee uses force or threats. Most states consider any sexual relationship a felony.

In one instance cited in Fine's report, a prosecutor refused to pursue a case against a teacher in a federal facility who confessed to having sex with an inmate. The unidentified prosecutor called the case a "stupid sex case" and a "waste of time" because it was a misdemeanor.

Even when cases are prosecuted, the punishment is inadequate, Fine said. A psychiatrist at a federal detention center was convicted of having sex with several female patients but received only a one-year sentence.

Fine said "federal laws need to be strengthened to provide greater deterrence to this criminal conduct." Federal laws don't even apply to sexual abuse of federal inmates in privately run prisons, which house 15 percent of all federal inmates, he said.

In the past five years, Fine's office has opened 351 investigations of alleged sexual abuse, about 12 percent of all investigations undertaken by the inspector general.

Thirty-one states set maximum prison terms of at least five years for sexual abuse (search) of inmates without any indication of force or threats, Fine said. Of those, eight have maximum terms of at least 10 years.

Officials of the Bureau of Prisons (search) did not immediately comment Monday.

Kathleen Hawk Sawyer, a former bureau director, has called sexual abuse of inmates the biggest problem she faced.