Police sources and union officials have raised questions about the methods used by a high-ranking D.C. police department panel investigating allegations of improprieties and possible criminal violations in the department's controversial drug screening program.

The report, which is expected to be presented to Chief Maurice T. Turner Jr. today, focuses on allegations brought last summer by two employes of the D.C. Police and Fire Clinic, including charges that test results were manipulated to allow one police official to win a promotion. Sources said the careers of several high-ranking officials could hinge on the report's findings.

In interviews with police officials, police union leaders and department critics, objections were raised about key elements of the investigation, which many view as a litmus test for the department's character at a time when several federal grand juries are investigating allegations of corruption among city officials, including some D.C. police vice squad members.

According to sources familiar with the investigation:

Clinic and police Internal Affairs Division officials, whose activities were potential targets of the probe, were allowed to read copies of confidential letters detailing the exact allegations. The letters, sent last summer to Mayor Marion Barry and U.S. Attorney Joseph diGenova, provided witnesses a blueprint of the inquiry before they ever were called to testify.

The staff of internal affairs was used to transcribe testimony from the closed-door sessions, even though the division is a key target of the investigation. The procedure has raised concerns over the confidentiality of the witnesses' testimony, according to sources, who said professional stenographers are normally used in such cases.

Substantial gaps have appeared in the transcribed testimony of some witnesses, who have been asked to fill in the blanks, but there were no such gaps in the transcription of panel members' questions.

Turner declined through a spokesman to comment on any aspect of the report or the drug screening program, and ordered members of the department not to discuss it.

Gary Hankins, labor committee chairman of the Fraternal Order of Police, in an interview, termed the three procedures "incredible circumstances."

He harshly criticized the panel, which is made up of two assistant chiefs and a general counsel for the police department, for showing witnesses the letters detailing allegations before they testified.

"That was a serious error in judgment," Hankins said. "By showing witnesses the letter it left the witness with the discretion of what he or she would comment on, and what he or she wouldn't comment on."

Despite concerns over the manner in which the investigation was conducted, informed sources said some disciplinary action is likely to be recommended.

Last summer a drug screening supervisor and test operator at the clinic went to Barry and diGenova with detailed allegations of improprieties and tampering with urine samples by clinic and police officials, including Internal Affairs Division officials responsible for investigating corruption among police and city officials.

The screening workers, Vernon Richardson and Marguerite Anastasi, alleged that after receiving numerous telephone calls from a top-ranking police official, a police lieutenant and a sergeant from internal affairs tampered with drug testing procedures on behalf of another lieutenant seeking promotion to captain.

In their letters, the clinic workers alleged that clinic officials and members of IAD manipulated test proceedures to obtain "desired results." The employes also alleged other violations, including bribery, tampering with physical evidence and violations of standards of conduct.

The letters also detailed a May 30, 1985, incident in which a lieutenant up for promotion to captain returned to the clinic in civilian clothes for a second drug test after a sample of his urine taken earlier that day had tested positive for marijuana on clinic testing equipment.

According to department regulations, offering a second chance at a drug test is prohibited.

Richardson was ordered to collect the second sample by clinic and internal affairs officials, after they received several calls from an unnamed but high-ranking police official, according to the letters.

The lieutentant's second urine sample was not tested at the clinic, as is routine, but was flown, along with four other samples, to a North Carolina laboratory by a police sergeant for a confirmation test, the letters stated. Normally, urine samples are tested at the clinic, and if traces of drugs are found, they are sent by overnight mail to the laboratory for confirmation.

When the sample was returned from the North Carolina lab to the clinic, it was reported free of drugs and the lieutenant received his captain's bars.

Richardson and Anastasi were briefly reassigned to lesser duties in September after their letters were shown to their clinic supervisors before those officials testified before the panel. That reassignment was reversed and now is part of the investigation.

In August, Turner appointed the three-member panel to investigate the allegations. The questions of who ordered the tampering and whether it was also done in other cases are central to the in-house investigation and to the integrity of the drug screening program, which has emerged as an increasingly important internal center of power that can exercise control over officers from the beginning of their careers to the end.

Drug screening tests are required of police recruits, sworn members up for promotion and those accused of substance abuse. The tests also are a routine part of yearly physical examinations given to officers 35 years and older.

Applicants who test positive for drugs are not hired. Police officers are subject to termination after one positive urine test is confirmed by an outside laboratory. At least 40 officers and 50 probationary officers -- those in their first year -- have lost their jobs because of allegedly tainted urine specimens since Turner started the testing program in 1982.

Serious flaws in the program similar to other allegations raised recently by the two clinic workers also were uncovered more than two years ago by the internal affairs investigators, including missing and altered documents, breaches of security and shoddy record keeping. It is unclear what, if any, action was taken to correct those deficiencies.

"The question to employes of the department and to citizens is can the integrity of the system be restored or does it have to be changed completely," said one police official, who asked not to be identified. "The answer lies in who ordered the second urine test and whether it was the exception or the rule."

City Administrator Thomas M. Downs, in an interview on Wednesday, seemed to distance himself and Barry from the report, placing the responsibility solely on Turner.

"Neither the mayor nor I can run the police department from the fifth floor of the District Building," Downs said. "When it comes to judging the actions of police officers and civilians within the department, it's the chief's responsibility."