Grant Wahl, one of the best known football writers in the United States, died early Saturday while covering the World Cup match between Argentina and the Netherlands.
US media seated near him said Mr Wahl fell back in his seat in the media tribune at Lusail Iconic Stadium during extra time and reporters adjacent to him called for assistance.
Emergency services responded very quickly but it was later confirmed that the journalist had died.
Wahl was covering his eighth World Cup and wrote on Monday that he had visited a hospital while in Qatar as he was feeling unwell.
And his brother Eric Wahl, who is gay, believes that the journalist could have been killed as he had been "healthy" and had "received death threats".
Grant had been stopped briefly by security, as he tried to enter the stadium for the US vs Wales group game, because he was wearing a rainbow shirt.
His brother Eric said through tears on Instagram: "I am Grant Wahl’s brother I am gay I am the reason he wore the rainbow shirt to the World Cup.
"My brother was healthy, he told me he received death threats, I do not believe my brother just died, I believe he was killed and I just beg for any help."
Grant had earlier at the World Cup written about feeling unwell: "My body finally broke down on me. Three weeks of little sleep, high stress and lots of work can do that to you.
"What had been a cold over the last 10 days turned into something more severe on the night of the USA-Netherlands game, and I could feel my upper chest take on a new level of pressure and discomfort.
"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. They gave me a course of antibiotics and some heavy-duty cough syrup, and I'm already feeling a bit better just a few hours later. But still: No bueno."
Mr Wahl wore a rainbow T-shirt in support of LGBTQ rights to the United States' World Cup opener against Wales on November 21 and wrote that security refused him entry and told him to remove the shirt.
Gay and lesbian sex is criminalised in Qatar, a conservative Muslim nation.
Mr Wahl wrote he was detained for 25 minutes at Ahmed Bin Ali stadium in Al Rayyan, then was let go by a security commander. Mr Wahl said Fifa apologised to him.
Following news of his death, the US Soccer Federation said in a statement: "We could always count on Grant to deliver insightful and entertaining stories about our game, and its major protagonists.
"Grant's belief in the power of the game to advance human rights was, and will remain, an inspiration to all. Grant made soccer his life's work, and we are devastated that he and his brilliant writing will no longer be with us."
US State Department spokesman Ned Price tweeted: "We were deeply saddened to learn of the death of Grant Wahl and send our condolences to his family, with whom we have been in close communication. We are engaged with senior Qatari officials to see to it that his family's wishes are fulfilled as expeditiously as possible."
Mr Wahl is survived by his wife, Dr Celine Gounder, an associate professor at New York University School of Medicine, attending physician at Bellevue Hospital Centre and CBS News contributor.
Dr Gounder tweeted: "I am so thankful for the support of my husband @GrantWahl's soccer family & of so many friends who've reached out tonight. I'm in complete shock."
A voter at times in FIFA's annual awards, Mr Wahl wrote this week that he had been among 82 journalists honoured by FIFA and the international sports press association AIPS for attending eight or more World Cups.
