A seven-month investigation of contracting practices of the agency that runs the District's juvenile institutions has been broadened to determine whether city officials or staff members altered or withheld documents subpoenaed by a federal grand jury, according to city and law enforcement sources.

The FBI, General Accounting Office and U.S. attorney's office are investigating nearly all aspects of the D.C. juvenile justice system that are controlled by the Youth Services Administration. The investigation is focusing on the agency's dealings with contractors and its overtime payments to its own employes, according to the sources.

Audrey Rowe, D.C. commissioner of social services, said yesterday that FBI agents and GAO investigators recently moved into the youth agency offices, which are in the Receiving Home for Children in Northeast Washington. The investigators have been using computers to analyze payroll records, have reviewed the agency's $5 million in outside contracts and have summoned employes from the city's juvenile institutions for interviews, Rowe said.

Youth agency employes are "having interrogrations for several hours," said Rowe, who said she was conducting her own internal review and that her agency was cooperating fully with federal investigators.

"You have so many different investigative bodies involved at looking at the same issues -- the FBI, GAO, [D.C.] inspector general, grand jury -- everybody's looking at the same issues and there's paper flowing everywhere," she said.

Patricia Quann, director of the Youth Services Administration, said, "If they find something, and somebody who has been breaking the law, I mean we certainly want that found out."

The investigation is one of several recent federal probes into contracting practices of the District government. An official of the GAO said a federal audit of District procurement procedures is expected to be completed in two weeks.

The Youth Services investigation was begun after congressional hearings last fall that examined whether the city violated federal laws concerning special education of children held in District institutions. The hearings prompted the U.S. attorney's office to open an investigation into possible mismanagement in contracts awarded to a private firm -- Educational Support Systems -- that does special education assessments for the agency.

The investigation was broadened after the GAO, a congressional watchdog agency, examined initial documents requested after the congressional hearings. The probe now covers all contractors who work with the District's juvenile institutions, group homes for delinquent youth, shelter houses for children who are awaiting trial and the community service program in which the agency supervises delinquent children in their homes.

An attorney for Educational Support Systems, F. Joseph Warin, has said the firm has followed all proper procedures in contracts with the city. The firm's director, Nancy Opalack, could not be reached yesterday.

District officials have complained that the scope and intrusiveness of the federal investigation has affected day-to-day agency operations.

Employes are spending much of their workday time retrieving and copying the many documents requested by investigators, according to Rowe and Quann. In one case, classes could not be held at Oak Hill, the maximum security institution for D.C. juveniles in Laurel, because teachers were recruited to photocopy documents, according to an Oak Hill official.

This week Rowe, who oversees the youth agency, was interviewed by both the FBI and GAO.

The federal investigators working in the Receiving Home have been comparing the agency's records with copies of the same records that they received last year in response to a federal subpoena. In some cases, they have found discrepancies, according to law enforcement sources, and in other cases have found documents necesssary to the investigation that were never turned over.