Brett Gardner's 14-year-old son, Miller, tragically died from carbon monoxide poisoning.

Costa Rican officials confirmed the cause of death after they said toxicology results showed the teen had a carboxyhemoglobin saturation of 64 percent ... well above the 50 percent threshold that's considered lethal.

Costa Rica's director of the Judicial Investigation Agency, Randall Zúñiga, stated the young Gardner likely came into contact with the gas through a machine room that was situated next to where the Gardners were staying while on vacation.

Miller passed away in his sleep on March 21 ... and, initially, authorities believed he may have died from food poisoning and subsequent asphyxiation. An autopsy quickly ruled that out, however, and authorities then turned their attention toward the potential of carbon monoxide poisoning.

Brett and his wife, Jessica, called Miller "a beloved son and brother" in the days after his passing ... and added they could not "yet comprehend our life without his infectious smile."

In an obituary posted last week, the Gardners said Miller loved football, baseball, golf, fishing, hunting, dirt biking and "traveling to new places with his family."

They encouraged people to send donations to the Make-A-Wish Foundation in his honor.

RIP.