A food poisoning outbreak that killed two people and sickened 51 has highlighted just how little is known about morel mushrooms and the risks in preparing the popular and expensive delicacy.

The FDA conducted an investigation into morel mushrooms after the severe illness outbreak linked to Dave’s Sushi in Bozeman, Montana, in late March and April. The investigation found that undercooked or raw morels were the likely culprit, and it led the agency to issue its first guidelines on preparing morels.

“The toxins in morel mushrooms that may cause illness are not fully understood; however, using proper preparation procedures, such as cooking, can help to reduce toxin levels,” according to the FDA guidance.

Even then, a risk remains, according to the FDA: “Properly preparing and cooking morel mushrooms can reduce risk of illness, however there is no guarantee of safety even if cooking steps are taken prior to consumption.”