WASHINGTON (AP) _ An investigation into cheating at the government's air traffic controller school in Oklahoma has determined that some test information was compromised, although the impact of the cheating is unclear, according to sources and documents.

The Transportation Department's probe, which has yet to be completed, has concluded that test material similar to actual test problems was in the possession of students and probably provided unfair advantages, it was learned Monday.

An analysis by the school's quality assurance office of a packet of problems surrendered by a student in March concluded ''the student had correct information pertaining to nine of the graded laboratory problems'' on which he was about to be tested, according to an internal memorandum obtained by The Associated Press.

''This means that nine of the 14 graded problems were compromised,'' supervisor Oliver Spires wrote in a memorandum dated May 8. He said that while some details of the test problem, such as flight numbers, were missing, the traffic ''situations'' facing the student were identical to those on some test problems.

Senior officials at the Federal Aviation Administration Academy outside Oklahoma City, nevertheless, maintained in interviews that while students over the years may have developed and circulated problems closely mirroring actual test situations, the school's testing program remains sound.

The laboratory tests, which have been the focus of the cheating probe, consist of a series of traffic management problems in which students must safely guide aircraft during a 30-minute period. A student is graded on five problems selected by instructors from a pool of problems. The performance test accounts for 60 percent of a student's grade.

''I am convinced that this (testing) program is not compromised. ... It's a performance type of situation that you cannot cheat on,'' Morris Friloux, the school's superintendent, said in an interview last week.

Friloux called the quality assurance office's analysis, which he had requested last March, ''purely opinion'' and declared: ''How can someone say a program is compromised when 50 percent (of the students) are not making it.''

On average, about 60 percent of the students pass the 13-week course, but during the last 12 months when test material supposedly was being circulated, the pass ratio fell below average four times and was barely 50 percent in May.

In three cases, the pass percentage exceeded 70 percent, but officials, citing higher aptitude scores, attributed the unusual success rate in those classes to a better quality of students.

Actual scores on the six laboratory problems were only slightly higher than average in 1986 and actually dipped several percentage points during the first part of this year.

Criticism of the FAA's controller testing program persists among some members of Congress, nevertheless.

Rep. Guy Molinari, R-N.Y., said he urged FAA Administrator Donald Engen to suspend testing of controllers until assurances can be made that none of the test problems is in the hands of students.

Molinari charged that the FAA and Friloux ''have closed their eyes to the problem.''

An FAA spokesman, Bob Buckhorn, said Engen was preparing a response to Molinari, which will include a reiteration that the agency does not believe the tests are being compromised. He characterized the documents in circulation as being more akin to ''student notes'' than actual test problems.

Buckhorn said questions have been raised about 10 problems in all. Eight already have been replaced, with the other two ''scheduled for replacement within the next 60 days,'' he said.

Douglas Murphy, manager of the FAA school's air traffic branch, maintained that the test information found in student hands could be the product of copius notes passed on by students over the years instead of a security leak.

According to several sources, who spoke on the condition they not be identified, investigators have been unable to pinpoint a single source for the test material found so far.

A student who voluntarily surrendered a packet of test material last March, several days before taking the examination, told investigators he had received it from another student who got it from a third student.