(ABC4) – The death of a Michigan teenager just days after he received his second dose of the COVID-19 vaccine is now under investigation.

Health officials say a 13-year-old from Saginaw County – located in central Michigan – died in his sleep in mid-June, three days after his second dose, according to the Detroit Free Press.

A joint statement from county and state health officials says the investigation into whether there is a correlation between the teen’s death and the vaccine is now at the federal level with the Centers for Disease Control and Prevention.

The boy’s aunt tells the Detroit Free Press that he had received the Pfizer vaccine at a Walgreens store. She describes her nephew as healthy with no underlying medical conditions, but he had complained of the common post-vaccine symptoms, like fever and fatigue.

According to The Detroit News, the investigation into what caused the boy’s death is going to take a “fairly long time.”

Health authorities are working to determine whether heart inflammation that occurs with many types of infections could be a rare side effect in teens and young adults after the COVID-19 vaccine, according to the Associated Press.

A recent report says several teenage boys across the U.S. have experienced heart inflammation after receiving a COVID-19 vaccine, but a link to the doses has not yet been made. But, according to that report, none of the boys were critically ill. In May, the New York Times reported the CDC is investigating the relationship between the rare heart problem and the COVID-19 vaccine.

ABC4 previously reported on a Utah teenager, 17-year-old Everest Romney of Draper, who had to be hospitalized for blood clots in his brain days after getting the COVID-19 vaccine.