By all measures, 20-month-old Luke Zachara was a healthy young boy with an infectious laugh and an undeniable bond with his big sister.
"He was the sweetest baby. We called him sweet little Lukie, sweet baby boy," Luke's mother Jordana Zachara told NBC 5.
But one tragic night in February, Luke passed away suddenly in his crib.
"The day started out pretty normal,” his mother said.
Jordana Zachara was getting ready for work when Luke’s dad, NBC 5 photographer Ron Zachara, went to wake him up in his crib.
It was then that Ron Zachara was faced with an unimaginable horror.
"[Luke] didn’t move," he said. "And he was cold."
"I still remember [Ron] saying call 911," Jordana Zachara said.
Despite their efforts to revive the child, the distraught parents were told it was too late. Luke was gone.
The Zacharas waited days to find out what happened to their healthy child.
"There was no distress through whole night through video and audio monitor," Ron Zachara said. "All those things as a parent, you're asking yourself it had to be something I did wrong."
But the family soon learned their unexpected answer: Sudden Unexplained Death in Childhood.
Hundreds of child deaths across the U.S. have been attributed to the medical mystery, but it’s a category of death many parents, including the Zacharas, had never heard of.
Glenview parents Estuardo and Raquel Torres said they know the diagnosis all too well. Their son Julian’s death was eerily similar to Luke’s.
"Our son was playing. He was happy and then fell asleep and then just didn’t wake up," Raquel Torres said.
Julian died about three years earlier, but the family said they’re still searching for answers.
"I feel like it gets harder, because now we’re living more than the years he was alive," Torres said.
Julian had been celebrating his oldest sister’s birthday the night before his unexpected death.
"No signs. He didn’t have a fever; there were no signs. He just went to sleep like a little angel and just didn’t wake up," Torres said.
Julian's death was also declared SUDC.
"There’s no answer," said Estuardo Torres. "So for us there’s no closure."
In Cook County, about 10 percent of child death investigations are labeled SUDC, said Assistant Medical Examiner Dr. Eric Eason.
"I can only imagine how frustrating it would be for a family to receive a death certificate that says that," he said.
SUDC is the fifth leading category of death for children ages 1 to 4, according to Laura Gould-Crandall, a research scientist with New York University and founder of the SUDC Foundation.
Gould-Crandall, whose 15-month-old daughter also died of SUDC, said she started the foundation to provide support for families as they continue to search for answers.
"Every physician that I spoke with said this is not a major problem," she said. "Fast forward to today, we have over 900 families registered with us from over 18 countries."
SUDC is not the same as Sudden Infant Death Syndrome, which impacts babies under a year old, experts said. But because few people know about it, there is no federal money going to SUDC research.
Eason is working on a book that will spell out SUDC guidelines.
"The ultimate goal is to prevent these, but people have to know about it first,"he said.
There’s also private research underway at NYU, which includes the creation of a genetic database of SUDC families.
"Doing the genetic makeup it’s almost just keeping the door inched open a little bit to get that answer to know what actually happened," Ron Zachara said.
The Zacharas said they know answers won’t rid them of their grief, but they are determined to make sure Luke’s life mattered.
"Hopefully something huge comes out of this whole mystery," Ron Zachara said.