A 27-year-old guard at the D.C. Jail was arrested last night on charges that he sexually assaulted a 17-year-old inmate in her cell, D.C. police reported.

Michael Roundtree, of 2652 Martin Luther King Ave. SE, turned himself in at D.C. police headquarters about 7:30 p.m., police said. They said he was charged with sodomy, a felony, in a D.C. Superior Court warrant.

Police said he would be held at the D.C. police department cell block pending arraignment in Superior Court today.

The arrest came after a 17-year-old girl, who was incarcerated at the jail on charges of soliciting prostitution after telling authorities she was an adult, told police that she was orally sodomized by a corrections officer against her will. The attack, she said, occurred in her cell on Sunday.

Under D.C. law, persons under the age of 18 and charged with a crime such as soliciting prostitution are not held at the D.C. Jail.

Juveniles can be held at the jail if they are charged as adults in more serious crimes such as murder.

Authorities said Wednesday that the girl's real age was discovered only after police started to investigate her allegations.

However, a source close to the case said corrections officers at the jail and the U.S. attorney's office were told at least three days before the alleged assault occurred that the girl was underage.

Sources said that sometime after the girl was jailed May 2, she told guards that she was 17 and immediately was moved to a protective custody unit for women in the jail's infirmary. The alleged assault occurred there, according to a source in the U.S. attorney's office.

The girl is currently being held in a receiving home for juveniles, said Charles Ogletree of the Public Defender Service.

When the girl was arrested for soliciting prostitution on April 30, she gave police a fictitious name and told them she was 19 years old. She appeared before Commissioner Thomas Gaye in D.C. Superior Court the next day and was released into the custody of a certified custodian, a source familiar with the case said.

Later that day, she was arrested for soliciting prostitution, and the next day, May 2, she again appeared before Gaye, who apparently was upset that she was before him twice in less than 24 hours on the same charge, the source said. She gave police and court officials the same fictitious name and date of birth as the first time she was arrested, the source said.

Gaye ordered her held at the D.C. Jail on $500 cash bond.

Sometime after her arrival at the jail, the girl told corrections officers that she was 17 years old and she was taken to a protective custody area of the jail for her own safety, the source said. A corrections department spokesman said last night that he could not confirm that information.

On May 8, her attorney interviewed her at the jail and learned her correct name and age, according to the source.

The attorney called the girl's father in Minneapolis and was told the girl was 17, the girl's father said in a telephone interview.

The lawyer went to the U.S. attorney's office on May 9 -- three days before the alleged assault -- and told them that the girl was a juvenile, the source said. He said that the U.S. attorney's office asked for a certified birth certificate before it would take action on the case.

A spokeswoman for the office refused to comment last night.

However, one prosecutor said, "When a person says she's 19, we charge her as an adult. When an attorney says she's 17, we say show us. Usually we require something other than just a change of mind."

Meanwhile, D.C. Mayor Marion Barry, at his monthly news conference, said, "I will never condone, if this is true, any action on the part of any correctional officer or any other employe that would sexually assault or abuse anybody in this city, whether you're in jail or out of jail, whether you're young orold . . . . "

And, Bernard Demczuk, political director of the National Federation of Government Employees, which represents correction employes, said that the department should do more thorough background checks and conduct psychological evaluations before hiring persons as correction officers.

"The union has been demanding and will continue to demand that the department have more stringent criteria and a better screening process before hiring employes," he said.

On Monday, Marion Strickland, the deputy director of the D.C. Department of Corrections, addressed roll call at the jail and delivered what one source called a "stern lecture to corrections officers about being professionals."

Strickland did not return a reporter's call yesterday.