Costa Rican authorities are actively investigating the tragic death of Miller Gardner, the 14-year-old son of former Yankees star Brett Gardner, the US State Department confirmed to The Post Monday. “We can confirm the death of a US citizen in Costa Rica on March 21,” a department spokesperson said in reference to the teen. “We are closely monitoring local authorities’ investigation into the cause of death. Out of respect for the privacy of the family and loved ones during this difficult time, we have no further comment at this time.” Miller fell ill “along with several other family members while on vacation” in the tropical Central American country, the Gardners said in a statement Sunday. The teen died at a resort in Costa Rica, Inside Edition reported Monday. Gardner, the speedy outfielder who played 14 years in pinstripes — including during the Bombers’ 2009 World Series championship — retired in 2021 and has since lived with his family in South Carolina. “We have so many questions and so few answers at this point, but we do know that he passed away peacefully in his sleep on the morning of Friday, March 21st,” the Gardners’ statement read. “Miller was a beloved son and brother and we cannot yet comprehend our life without his infectious smile. He loved football, baseball, golf, hunting, fishing, his family and his friends. He lived life to the fullest every single day. “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. Our prayers go out to Miller’s teammates and friends, as well as to all other families who have lost a child far too soon as we share their grief.” Miller’s shocking death sparked an outpouring of support, including from Gardner’s former minor league teammate and Mets manager Carlos Mendoza. “I had a really good relationship with [Gardner] and we’re thinking about him,” a choked-up Mendoza said. “The whole Mets organization is thinking about the Gardner family. “I got two boys, too, and they were pretty close.” The Yankees offered their own condolences to the family in a heartfelt statement. “Our hearts are heavy, and the Yankees family is filled with grief after learning of the passing of Miller Gardner,” the team said. “Words feel insignificant and insufficient in trying to describe such an unimaginable loss. It wasn’t just Brett who literally grew up in this organization for more than 17 years — so did his wife, Jessica, and their two boys, Hunter and Miller.”