The U.S. Food and Drug Administration has warned Medtronic Inc. (MDT) about failing to properly report and handle problems with its Synchromed drug-infusion pump and its MiniMed insulin pump.

In a June 1 warning letter, the FDA said it took Medtronic almost two years to recall its Synchomed II Pumps after discovering a missing propellant in some of the products.

"FDA is concerned with your failure to initiate a recall ... in a timely manner," the agency said in the letter, posted on the FDA's Web site Wednesday.

The agency also said the company didn't follow proper reporting procedures involving complaints related to the MiniMed insulin pump.

The warning letter was the result of an FDA inspection of a Medtronic plant in Puerto Rico conducted in November and December of last year.

In a statement, Medtronic said it was "working diligently to respond to the FDA on all of the issues and to ensure that any additional actions necessary to resolve the concerns are in place as quickly as possible."

The bulk of the letter relates to the company's Synchromed drug infusion pump and failures to comply with agency good-manufacturing-practice standards. The company started formal correspondence with the FDA in January aimed at correcting the manufacturing violations found during the inspection.

The FDA said "multiple" pumps were released and implanted in patients even though they weren't filled with a propellant needed for the pump to operate correctly.

The FDA said the company first identified a problem with the pumps in May 2006, and then in January 2007 the company determined the cause to be related to a failure to properly fill propellant into the pump's reservior. The devices were recalled in May 2008.

The FDA also said there was a failure to verify the fill weight of the devices after being processed through fill equipment. Had the company followed such procedures, it might have detected a manufacturing problem before the devices were released to the public.

The agency noted that Medtronic has retrained employees on the Synchromed manufacturing process and that the company has established additional "checkpoints" in the manufacturing process. The FDA said it would determine the adequacy of such steps during the next plant inspection.