D.C. Police Chief Maurice T. Turner Jr. defended the integrity of his department yesterday, saying that a federal grand jury investigation of alleged corruption in the 4th District vice squad centers on "an isolated case" of wrongdoing among a handful of officers.

Turner said the U.S. attorney's office has assured him that there is "no widespread corruption in the police department," and that the grand jury investigation whether officers kept money and drugs seized during raids is "on a fast track because we need to put this behind us and get on about the business of providing public safety for the city."

At the same time, Turner said that the U.S. attorney's office, which launched an undercover investigation of the 4th District vice unit without informing him, has not "dealt fairly with me" in the probe. Turner would not elaborate, saying he will make a public statement about the handling of the investigation at its conclusion.

In a wide-ranging, 90-minute interview with Washington Post reporters and editors, Turner also touched upon a variety of other problems that have shaken the police department in recent months, including allegations of improprieties in the department's urine testing program and charges that some officers in the 4th District lied in affidavits in support of warrants and may have leaked information to drug dealers before the February 1986 Operation Caribbean Cruise drug raid.

Turner, in addition, expressed opposition to the early release of violent offenders from the city's prisons and, sounding a theme he has emphasized thoughout his six-year tenure as police chief, said that widespread drug abuse in the District has fueled crime, especially among juvenile offenders.

The chief, cautioning that juveniles pose a growing threat to public safety, said that some juveniles are incarcerated only after they have been held responsible for two or three felony offenses and suggested the city should take a harder line with them.

"I just don't believe that the general public is being well served in the release of these juveniles," Turner said, adding that a population ceiling ordered recently by a Superior Court judge for three District-run juvenile facilities would lead to greater numbers of juvenile offenders on the city's streets and higher levels of crime.

In his most detailed public comments about the federal probe of the police department, Turner asserted that "the department has continued to function" despite the investigation. But he warned that large amounts of money from illegal drug sales in the city have increased the temptations for police officers and created greater potential for corruption.

"I am concerned about police officers socializing with known drug dealers. It's a violation of our rules and regulations . . . and certainly we would want to try to identify those individuals who are violating" those rules, Turner said.

In the interview Turner focused on three main issues:

Alleged skimming of drugs and money by members of the 4th District vice squad and charges that some members of the unit lied in affidavits supporting search and arrest warrants.

Because of the allegations, U.S. Attorney Joseph E. diGenova announced last month that his office would drop from 300 to 400 drug cases that had been investigated by members of the squad since January 1986, and Turner reassigned the entire 16-member unit.

Mayor Marion Barry and some members of the D.C. Council sharply criticized diGenova for dropping the cases, but Turner said yesterday that he "concurred completely . . . with the recommendation that those cases be dropped and I still concur with that position, in spite of comments from other government officials."

He said the department is considering new "management controls" to strengthen oversight of narcotics investigators -- who now number nearly a quarter of the 3,880-member force -- and new procedures for obtaining and executing warrants. Those controls will include use of a computer and master log to track all warrants obtained by D.C. police citywide, Turner said. He said more supervisors now will be required to be on the scene when warrants are executed.

The skimming allegations were taken to the U.S. attorney's office by police officers who complained that the department was apparently unwilling to investigate the charges. Turner said that neither he nor anyone else in the department, to his knowledge, was informed about the allegations before the officers went to the U.S. attorney.

In a related development, U.S. District Judge John Lewis Smith has signed orders overturning the drug convictions of Rupert G. Morris and Farron L. Brown after the U.S. attorney's office moved to vacate the charges against them. It was the first time that convictions in a drug case investigated by members of the 4th District vice squad were overturned. The case was investigated by Shelton D. Roberts, one of two officers from the unit who has had his police powers revoked pending completion of the federal probe.

Allegations by two employes of the department's drug screening program that officials of the program manipulated test results.

Turner appointed a three-member panel to investigate those allegations and the circumstances surrounding a May 1985 incident in which a lieutenant and a sergeant in internal affairs allegedly tampered with drug test results on behalf of a lieutenant who was up for promotion.

Turner said yesterday that he has "complete faith and confidence" in the program, adding that he has not read the panel's report but that he expects it to be completed this week.

"Basically, I'm told that they {the panel} say it's a good system," Turner said of the program, which screens members of the force and recruits for drug use. "Obviously there were some things that happened that shouldn't have happened and we're going to call a spade a spade and take action."

Irregularities similar to those alleged by the two employes -- including instances of missing and altered documents and breaches of security -- were uncovered more than two years ago by an Internal Affairs Division investigation of the program.

Turner, asked whether the agency's findings were acted upon, said that some changes were made "to correct what was spelled out" in the internal affairs report. He added, however, that if recommendations made in the internal affairs report were not implemented, he expects the report from the three-member panel to explain why. Allegations that some members of the 4th District vice squad leaked information to targets of Operation Caribbean Cruise, a massive drug crackdown staged by the 4th District police that fell far short of expectations.

Turner said that, after listening to secretly recorded converstations in which the drug dealers identified police officers who had tipped them to the raid, he allowed Deputy Chief James P. Shugart to investigate the information because Shugart "wanted a chance to redeem himself." Shugart was commander of the 4th District at the time of Caribbean Cruise.

When Detective Curtis Arnold, the investigator assigned to the case, was not able to confirm the information, Turner said he ordered Shugart to turn the investigation over to internal affairs. Shugart has said that Lt. Robert H. Drescher ordered Arnold to forward the investigation to internal affairs, but acquaintances of Arnold have said that he claims he was told to drop the investigation.

Turner said yesterday that "we have an internal investigation into what transpired" between the two.