EL PASO, Texas (AP) _ The FBI said Tuesday it is investigating an informant's tip that classified documents were found in U.S. Defense Department desks sent to a federal prison to be refinished by inmates.

The FBI received information that a prisoner at the medium-security La Tuna Federal Correctional Institution apparently had some of the documents, but he did not produce any when interviewed earlier this month, said agent Matt Perez.

Perez refused to disclose details of the continuing investigation, including the name of the inmate who was interviewed, but did say the FBI would conduct more interviews ''as soon as we can.''

La Tuna warden D.J. Southerland said he was unaware of any investigation.

''All I can say is officials at ... La Tuna have no knowledge of classified documents of any sort being found in furniture delivered at this facility for refinishing for other federal government agencies,'' Southerland said.

The warden said 40 percent of La Tuna's 825 prisoners, located in the prison and a satellite camp for trusties, are serving sentences on drug- related charges. Another 20 percent have been convicted of immigration law violations, he said.

Southerland said he has not initiated any investigation of his own, because ''I don't have any idea whether it's true or untrue ... I don't really know that I've got a problem.''

Pentagon and Air Force officials declined comment.