A drug-resistant superbug infected 32 people at a Seattle hospital over a two-year period, with the bacteria spreading through contaminated medical scopes that had been cleaned to the manufacturer's recommendation, officials said on Thursday.

Eleven of the patients infected at Virginia Mason Medical Center between 2012 and 2014 eventually died, the hospital and city health officials said. But those patients were critically ill before being infected and it was unclear what role, if any, the bacteria played in their deaths.

The patients were infected with drug-resistant bacteria, including the rare Carbapenem-resistant enterobacteriaceae, which are difficult to treat because they have high levels of resistance to antibiotics, said Dr. Jeffrey Duchin, a senior official at Public Health - Seattle & King County.

The report follows similar incidents in Pittsburgh in 2012 and Chicago in 2014, where contaminated endoscopes infected dozens of patients, health officials said. No fatalities were directly linked to the infections.

In the Seattle case, public health officials said the germs apparently spread from patient to patient by endoscopes used to treat liver and pancreatic illnesses. Duchin said the scopes are typically used for thousands of procedures each year in U.S. hospitals.

The scopes at Virginia Mason Medical Center were sterilized to existing standards before each use, public health officials and the hospital said.

"This is a national problem," Virginia Mason Medical Center said in a statement. "We determined that the endoscope manufacturer's, as well as the federal government's, recommended guidelines for processing the scopes are inadequate."

Duchin said it took investigators months to pinpoint the contamination, and the hospital has since instituted a rigorous decontamination process that exceeds national standards.

There are three major manufacturers of the scopes, called duodenoscopes: Olympus Corp, Fujifilm and Pentax. Their disinfection recommendations were approved by the U.S. Food and Drug Administration.

The FDA said in a statement it was actively engaged with manufacturers and other government agencies to develop solutions to minimize patient risk.

Olympus, which supplies many of the Seattle hospital's scopes, said in a statement it was "monitoring this issue closely."

It was unclear how many people were exposed to the superbug, officials said. The bacteria can cause serious infections such as pneumonia, bloodstream infections and meningitis.

Neither the hospital nor local health officials notified the public about the outbreak because "there was not a strong rationale for doing so," Duchin said.