A grand jury will investigate cases of rapes and sexual assaults that occurred in the Prince George's County Detention Center during the past few years, Arthur A. Marshall Jr., state's attorney, said yesterday. Robert C. Bonsib, an assistant state's attorney, has been appointed special prosecutor in the investigation.

The county grand jury will examine 12 cases described in a series of articles in The Washington Post in which rapists told how they had committed their crimes and male victims related details of the assaults. The rapists and most of the victims were identified by name.

"When you have statements of crimes in which people admit to criminal behavior on the front page of the newspaper, you have to do something," said Marshall. "Somebody has to look at it and I don't know who else would but a grand jury."

In most of the cases in the newspaper series, the victims of rape or sexual assault did not bring charges because they said they were afraid jail guards could not protect them from retaliation from the rapists or the rapists' friends. In other cases, victims said jail guards or police had discouraged them from bringing charges against the rapists. One victim said he did not want to testify in court about the rape.

Most victims of rapes and sexual assaults in the jail have not been convicted of any crime. They are awaiting trial, some on such charges as shoplifting, driving while intoxicated and trespassing.

The grand jury, convened April 1 on other matters, was scheduled to adjourn this week but yesterday Marshall appeared before the panel and asked its 23 members to extend their term. All agreed, Marshall said.

Another grand jury will be sworn in next week to make the usual decisions on indictments. This is the second time in 20 years that a special grand jury has been called, Marshall said.

Special prosecutor Bonsib, a 1973 Catholic University law school graduate who usually prosecutes rape and murder cases for the county, will be aided by investigator Charles Kiess and by members of the police department, Marshall said.

Marshall said the grand jury will be free to investigate other sexual assaults and rapes at the jail -- as well as other assaults at the jail -- although it is not required to do so.

The grand jury took a tour of the jail on June 2 because by statute each grand jury must examine jail conditions, Marshall said. In its report on the jail, issued yesterday, the jury found that the facility was overcrowded and "filthy," that "some inmates were being fed in the sleeping/living areas," and that "many areas of the jail were not under the direct observation of uniformed guards."

Marshall said he asked this grand jury to investigate the sexual assault cases because the panel already is familiar with the jail's problems.

Marshall also wrote a letter to County Executive Lawrence J. Hogan, who appoints the jail director, in which he urged Hogan to persuade county residents to vote for a bond issue Nov. 2 that would finance construction of a new county jail.

"I would urge that you assist in making the public aware of the problems that exist in the Prince George's County jail," Marshall said.

Hogan said he will address the problem of jail rapes and sexual assaults at a press conference today.