A 21-year-old man has been arrested as Santa Rosa police say they believe he is linked to the deaths of two high school students who possibly died from a fentanyl overdose.

Two Santa Rosa high school students have died after allegedly overdosing on fentanyl, Santa Rosa police said. Two other students remain hospitalized after also allegedly overdosing on fentanyl in a separate incident Saturday.

After a yearlong investigation, police charged a woman with second-degree murder after her child swallowed a fatal dose of fentanyl while they were living in Mooresville. Even a pencil tip amount of the synthetic opioid pain reliever drug can be fatal, according to the U.S. Drug Enforcement Administration.