WHEELING, Ill. (CBS) — Students and staff in the northwest suburbs were left with questions Sunday after the sudden death of their classmate.

School district officials said the student at Wheeling High School likely died of COVID-19. But his loved ones told CBS 2’s Steven Graves he tested negative for the novel coronavirus.

Now, they are warning others.

Zach Leviton was a healthy 16-year-old by all accounts. But the Wheeling High School sophomore started feeling ill last month.

His mother told CBS 2 over the phone that the symptoms were mild and there was no apparent sign of COVID-19.

But days passed and Zach got worse last Monday – with limp limbs and mild shortness of breath.

He was rushed to the hospital last week to get a COVID-19 test. But his mother said it came back negative, and the teen died after being on a ventilator.

She said her doctor told her there was no conclusive cause of death, but that Zach had heart complications and that he might have been suffering from the early stages of the virus.

Zach’s mother is now concerned about testing, and how the virus appears in young people.

COVID-19 usually attacks the respiratory systems of those with underlying conditions, especially the elderly.

But recent medical reports have come out suggesting the heart can also be affected.

And with a mystery surrounding Zach’s death, Arlington Heights-based Township School District 214 has since sounded the alarm to other parents.

“It is my unfortunate responsibility to inform you of the passing of one of our students this week, likely due to complications related to COVID-19,” Superintendent David R. Schuler said in a letter to parents.

Schuler also wrote that another student is currently in intensive care due to COVID-19. He did not specify which school that student attended.

“Our student services staff is working with family and friends to provide support during this tragic time,” Schuler wrote.

The superintendent also urged the school community to continue social distancing.

“While we may be seeing a plateau in confirmed cases and deaths related to COVID-19 in Illinois, this is a dark week in District 214 that leaves me with an incredibly heavy heart,” he wrote. “During this dark and painful week in D214, I would implore you to please follow the Governor’s stay-at-home order, wash your hands and practice social distancing.”

Meanwhile, loved ones are waiting for answers about Zach’s death through an autopsy.