Drug overdoses took a deadly toll in Santa Rosa over the weekend. Police are investigating the deaths of four young people, and the hospitalization of two more, to see if they are connected to a man arrested for selling narcotics,

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.