IN A LETTER to the editor today, U.S. Attorney Carl S. Rauh takes sharp exception to what we said last Sunday about the murder of Gloria Whipple. The essence of his complaint is that we misdirected our criticism of the failure of the legal system to keep Roy Anderson Leasure, one of those charged with beating Mrs. Whipple to death, off the streets. Mr.Rauh says the fault lies in the law that narrowly limits the use of preventive detention, and not, as we asserted, in his office and the court system.

Mr. Rauh is right on one point. We used a layman's term -- robbery -- to describe the crime for which Mr. Leasure had been previously convicted. Technically, he was convicted of assault and petty larceny (misdemeanors) for beating a high school student and stealing her coat. That distinction does make a substantial difference in the burden the government must carry when it seeks preventive detention of a rape suspect, which Mr. Leasure was at the time of the Whipple murder. But it does not make preventive detention impossible, although it might have been difficult to obtain in this case.

Indeed, the question of why preventive detention of Mr. Leasure was not sought was brought first to our attention by a member of Mr. Rauh's staff. Our concern was then heightened when inspection of court transcripts showed the possibility of such detention was raised by both the judge and the bail agency when Mr. Leasure first appeared in court on the rape charge.

Contrary to what Mr. Rauh asserts, we did not criticize the court system for failing to revoke Mr. Leasure's probation (or for failing to detain him). What we criticized was the failure of the system to have a judge consider any step that might have kept him off the streets. While we might not have raised the probation matter at all -- or pushed the preventive detention issue so far -- if we had known the rape charge against Mr. Leasure is as weak as Mr. Rauh now suggests it is, we were led to believe by his staff that this case was quite strong.

Also contrary to Mr. Rauh's letter, we have not altered our position on preventive detention. We have supported efforts in the past to modify that law in the direction sought by Mr. Rauh, and we expect to do so in the future. But we were not concerned in this instance with changes in the law. We were, and are, concerned with the failure to attempt to use its existing provisions in a situation where their use seemed -- on the basis of what we had been told -- compelling.