Grant Wahl, the celebrated soccer journalist who died suddenly last week at the World Cup in Qatar, had a rupture in a blood vessel leading from the heart, his family announced on Wednesday.

His death resulted from a weakness in an artery wall called an aneurysm, which may balloon outward and then tear open. An autopsy conducted in New York revealed that Mr. Wahl, 49, experienced a catastrophic rupture in the ascending aorta, which carries oxygenated blood from the heart.

The autopsy puts an end to rampant speculation that followed Mr. Wahl’s death. Posts on social media hinted at links to Covid vaccines or retaliation by the Qatari government for an article Mr. Wahl had written about immigrant deaths.

Mr. Wahl’s wife, Dr. Celine Gounder, is a leading infectious disease physician who rose to prominence during the coronavirus pandemic and advised President Biden’s transition team on Covid-19. She and the rest of the family rejected, in particular, the speculation linking his death to vaccines, saying that it was especially insulting because of her work.

He probably died instantly and did not feel pain, Dr. Gounder said in an interview on Tuesday. “I really do feel some relief in knowing what it was,” she said.

Mr. Wahl had been sick with a cold for several days before collapsing, and had written in his newsletter and on Twitter that he felt his body was breaking down after weeks of poor sleep and long days covering the games.

He had just turned 49 and was quite healthy, making his death even more of a shock to his friends, family and readers. The sniffles and other cold symptoms he had were most likely unrelated to the aneurysm, Dr. Gounder said.

Until the autopsy, Dr. Gounder said, she had been worried that perhaps she could have prevented his death if they had talked more often while he was in Qatar or if she had been there with him.

Mr. Wahl’s brother, Eric Wahl, initially said on social media that he suspected foul play and later suggested that his brother might have experienced a blood clot in his lungs. On Tuesday, Eric Wahl said that he no longer believed those were factors in his brother’s death.

The autopsy found that Mr. Wahl had an ascending thoracic aortic aneurysm, a weakening of the blood vessel that often goes undetected. As the aneurysm grows, it may produce a cough, shortness of breath or chest pain, some of which the doctors consulted by Mr. Wahl in Qatar might have attributed to his cold and a possible case of bronchitis.

In rare cases, the aneurysm can rupture and lead to death. Doctors are now exploring whether Mr. Wahl had Marfan syndrome, a risk factor for this type of aneurysm. He was tall and thin and had long arms, all of which can be signs of the genetic syndrome.

Mr. Wahl joined Sports Illustrated in 1996 as a fact checker, a traditional entry route for young journalists, and wrote hundreds of articles on a variety of sports for the magazine over the next two decades.

One early profile, a cover story on a teenage LeBron James in 2002, remained a touchstone for both writer and subject 20 years later. Mr. Wahl would occasionally reminisce about it to his 850,000 followers on Twitter, and Mr. James spoke about its meaning to him and his family while eulogizing the writer at a news conference and on social media over the weekend.

But Mr. Wahl was best known for writing about soccer, which he began covering while he was a student reporter at Princeton University in the early 1990s. Through his books, tweets, podcasts and magazine articles, he became a guide of sorts for a generation of fans and readers just learning the game.

He also used his profile and social-media megaphone to highlight the growth of women’s soccer, the breadth of corruption in soccer, human rights violations and gay rights.

Mr. Wahl had worked at Sports Illustrated for more than 23 years when the magazine’s publisher abruptly fired him over a dispute about pandemic-related pay cuts. But he had a large following by then, and started an email newsletter and podcast that quickly became successful.

In Qatar, Mr. Wahl was covering his eighth World Cup. He was in the press box in the closing minutes of the quarterfinal match between Argentina and the Netherlands when he collapsed.

According to two New York Times journalists who were present, medical personnel tried to revive Mr. Wahl for about 20 minutes before he was transported to a hospital in Doha. He was pronounced dead at the hospital.

Dr. Gounder’s relationships with the Biden administration and public health agencies, including the New York City health department, helped her bring the unembalmed body to the United States for the autopsy.

Dr. Gounder said she wanted to ascertain the circumstances of her husband’s death in part to quell online speculation. “I wanted to make sure the conspiracy theories about his death were put to rest,” she said.