One year later, mysteries remain about a deadly U.S. outbreak of fungal infections that were traced to steroid medications produced at a moldy Massachusetts compounding pharmacy.

Why did Tennessee end up with 153 infections and 16 deaths, when Minnesota received a similar amount of bad medicine and reported just 12 cases and one death?

Maybe Tennessee clinics left the injectable steroids on the shelf longer and they grew worse.

Maybe certain lots had more fungal growth than those shipped to the MAP pain clinics in the Twin Cities.

Answers about the outbreak, which was associated with 751 cases and 64 deaths overall, might not be forthcoming, said Richard Danila, an epidemiologist for the Minnesota Department of Health.

"All of that is very hard to ferret out, because there [were] multiple clinics involved and not every one was recording [information] in the same way."

Comparing the lots of drugs administered to patients and the timing of those injections would be helpful, but some clinics didn't record lot numbers or other key information in patient records.

Federal officials do expect to figure out why some patients were more likely to suffer meningitis or strokes than others, and which treatments worked best.

The University of Alabama at Birmingham has received a grant to follow as many as 500 patients through August 2015 and monitor long-term impacts of the infections on their health.

While Minnesota had only 12 confirmed infections, state officials received many more reports in fall 2012 of patients who reported symptoms that they feared were from tainted injections.

Reports of rapid declines in health added to the anxiety of the outbreak.

"Notifying people was a race against time," Danila recalled.

Some had never received the tainted drugs, though, while others were experiencing symptoms consistent with pre-existing chronic conditions and pain.

The one death, Danila said, involved a patient infected at a Minnesota clinic who died in another state of causes unrelated to the injection.