Investigators continue to probe a Medtronic plant that manufactured a small number of implantable defibrillators with a potentially serious problem, warning letters released last week by the Food and Drug Administration show.

The manufacturing problems at the plant in Juncos, Puerto Rico, that led to the recall happened in the months after Hurricane Maria's devastating sweep across the island last year, but a Medtronic spokeswoman said the issues were not storm-related.

In January, Medtronic sent out its first alerts to doctors and hospitals saying that 48 advanced Puerto Rican-made implantable defibrillators had a potential problem with electric arcing inside the device, and doctors should strongly consider removing them from some patients' chests.

Hundreds of other defibrillators affected by the same issue were pulled out of stockrooms and returned to Medtronic.

In total, the recall affected 951 devices, making it one of the smaller voluntary actions among recent heart device recalls. No one was killed or injured, and there was one recorded "field failure" of a device that led to loss of function, according to Medtronic's recall notice to doctors.

Some patients did undergo surgery to remove their devices prophylactically. Medtronic had advised a careful weighing of risks, as the small risk of mortality from the issue was comparable to that of replacement surgery.

"The warning letters [released last week] do not impact patients or physicians, and no new action is required by them, as all potentially affected devices related to this issue were voluntarily recalled in January and March 2018," said an Aug. 12 company statement from spokeswoman Kathleen Janasz.

The two warning letters show that regulators don't consider the issue resolved. FDA officials labeled defibrillators from the Juncos factory as "adulterated" in the Aug. 23 letters, and said inspectors would have to visit the site again to "verify compliance."

But the agency did say that the plans Medtronic set into motion since the inspections in April and May appeared to be "adequate" to address the problem that created the potential for electric arcing inside implanted defibrillators.

"The FDA stated that the Medtronic corrective action plan appears to be adequate to address the issues. At this time, Medtronic does not expect the warning letters to result in material negative financial implications to the Cardiac Rhythm and Heart Failure division or to Medtronic, and the company is working expeditiously to resolve the issues in the warning letters," the e-mailed statement said.

The warning letters say Medtronic's Puerto Rico plant, and the Minnesota-based CRHF division that oversees it from the Twin Cities, made a change in the manufacturing process that caused a defect at a point in assembly where it couldn't be retested before being shipped. The warning letters also say the change wasn't documented correctly.

Medtronic's recall notice for doctors said affected devices could have "an out-of-specification internal gas environment" inside them. That may create a risk of "internal arcing" during recharging of the device, which could cause it to stop defibrillating and pacing the heart.

Asked what "arcing" means, Janasz said via e-mail that it's when "an electrical path goes through the environment [for example, it 'jumps' through the air], instead of going through the intended electrical pathways."

The Class 1 recall, the most serious type, applied to model numbers of many of Medtronic's most advanced implantable heart devices, including several with sought-after compatibility with MRI scanners. The recall included various models of defibrillators: Evera and Visia ICDs, Viva CRT-Ds, and Amplia, Claria and Compia MRI CRT-Ds.

An ICD is a standard implantable defibrillator that monitors heart rhythm and can deliver high-voltage shocks to restart a regular heart beat and low-voltage pulses for pacemaking. A CRT-D is a defibrillator that performs "cardiac resynchronization therapy" by precisely delivering electricity to both sides of the heart to treat heart failure.

"Medtronic continues to strive for higher quality throughout the organization," the company statement said, "by enhancing its systems, processes and accountability, and is committed to the highest quality for its products and therapies. Medtronic is working closely with FDA to resolve the outstanding issues."