Santa Rosa authorities announced that at least two teens have died and two others were hospitalized following possible fentanyl overdoses over the weekend, with police making an arrest.

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.

Four teenagers in Santa Rosa overdosed on suspected fentanyl over the weekend, and two died, police said Sunday night.