The suspected drug dealer in a fentanyl investigation is expected to face charges Tuesday morning, after 2 high school students were found dead in a home and 2 other students were hospitalized for fentanyl overdoses at separated location.

Their sudden passing has prompted an outpouring of grief and raised urgent concerns about the dangers of fentanyl-laced drugs. As investigators work to piece together what happened, here’s what we know so far.

The Santa Rosa Police Department is urging parents to talk to their children about the dangers of fentanyl, which is often mixed with other drugs, leading to unintentional overdoses. Even as little as 2 milligrams of fentanyl can be fatal, officials warned.