Well-known football journalist Grant Wahl died suddenly after collapsing while covering a match at the World Cup in Qatar, his agent said.
US Soccer said it was "heartbroken to learn" of Wahl's death.
His wife responded to the US Soccer statement on Twitter, saying she was "in complete shock".
Qatar's World Cup organisers, the Supreme Committee for Delivery and Legacy (SC), paid tribute to Wahl's "enormous love of football" and offered condolences to his family, friends and media colleagues.
"Grant was known for his enormous love of football and was in Qatar to cover his eighth Fifa World Cup," the spokesperson for the Supreme Committee for Delivery & Legacy said.
"He fell ill in the Lusail Stadium media tribune, during last night's quarter-final match between Argentina v Netherlands. He received immediate emergency medical treatment on site, which continued as he was transferred by ambulance to Hamad General Hospital.
"We are in touch with the US Embassy and relevant local authorities to ensure the process of repatriating the body is in accordance with the family's wishes."
Attempts to revive Wahl at the stadium unsuccessful: agent
Wahl, a former Sports Illustrated sportswriter who moved to the Substack online publishing platform, had been tweeting about the Netherlands-Argentina match earlier on Friday.
His agent, Tim Scanlan, told Reuters that Wahl had "appeared to suffer some kind of acute distress in the start of extra time" at the quarter-final match.
Scanlan said attempts were made to revive Wahl in the press box before he was taken to a local hospital, where he was confirmed dead.
"Everyone's emotional and it's really traumatic," Scanlan said. "He was a true advocate for both the men's and women's games and really just cared deeply about the sport. He was empathetic and just truly a brilliant writer."
FIFA among those posting tributes
Gianni Infantino, president of world football governing body FIFA, offered condolences, saying in a statement that Wahl's "love for football was immense and his reporting will be missed by all who follow the global game".
Wahl said in late November he was briefly stopped at a World Cup stadium security screening point when he tried to enter while wearing a rainbow shirt in support of the LGBTQ community. In Qatar, same-sex relations are illegal.
He said World Cup security denied him entry to the United States' opener against Wales at the Ahmad Bin Ali Stadium in Al Rayyan and asked him to .
Wahl wrote on Monday that he had visited a hospital while in Qatar.
"I didn't have Covid (I test regularly here), but I went into the medical clinic at the main media center today, and they said I probably have bronchitis," he posted on Substack.
US State Department spokesman Ned Price said on Twitter the department had been in close communication with Wahl's family.
"We are engaged with senior Qatari officials to see to it that his family’s wishes are fulfilled as expeditiously as possible," said Price.
The US football community shared in an immediate outpouring of grief over the news.
"He was a kind and caring person whose passion for soccer and dedication to journalism were immeasurable," Major League Soccer Commissioner Don Garber said.
"This is so devastating," twice World Cup winner Megan Rapinoe said on Twitter. "All the love to his family and loved ones."
Writers and creatives who had worked alongside Wahl over the years said they were devastated, with TV producer Franklin Leonard saying it "is utterly shattering".
Others shared stories of their interactions with the journalist, saying he was extremely generous and kind.
