Miller Gardner -- the 14-year-old son of former New York Yankees outfielder Brett Gardner -- died from carbon monoxide poisoning, Costa Rican authorities have said citing a new toxicology report.
Authorities conducted a blood test for carbon monoxide and found a saturation level of 64% in Gardner's body, said Randall Zuniga, director of the Costa Rican Judicial Investigation Department. Concentrations above 50% are considered lethal.
Tests for other toxic substances -- and drugs like fentanyl -- came back negative, Zuniga said.
The case into Miller Gardner's cause of death is now closed, Zuniga said.
Miller Gardner died in his sleep on the morning of March 21 while on a family vacation. The hotel where Gardner died has not yet commented on the toxicology report, but previously said that high levels of carbon monoxide detected were in an adjacent mechanical room, not Miller Garnder's.
The night before the teenager's death, the family went out to dinner at a restaurant close to the resort and said they felt sick when they returned to the hotel, according to a Costa Rican source close to the investigation.
A hotel spokesperson said the staff "dispatched a medical team immediately, including a licensed doctor, which arrived to handle the emergency situation."
"Miller was a beloved son and brother and we cannot yet comprehend our life without his infectious smile," the Gardners said. "We are so thankful to all who have reached out to offer support and encouragement during this difficult time and we are confident our faith, family, and friends will help us navigate this unimaginable loss."
ABC News' Emily Shapiro contributed to this report.
