By Zhaoyin Feng, BBC Chinese Service, Washington
Bing Liu, a 37-year-old assistant professor at the University of Pittsburgh School of Medicine, was found dead in his house on Saturday.
Colleagues said he was close to making "significant findings" on his studies of Covid-19, sparking online speculation it was an assassination.
But police say it was a murder-suicide.
Why did Liu die?He was found with multiple gunshot wounds to the head, neck, torso and extremities at his home in the Pittsburgh suburbs, according to the local police department.
The gunman was identified as 46-year-old software engineer Hao Gu. Authorities say he took his own life after returning to his car.
Liu and Gu knew each other, homicide detectives said.
The investigation has determined it was a murder-suicide resulting from "a lengthy dispute regarding an intimate partner".
They said there was "zero evidence" the case was linked to Liu's research work and the current public health crisis.
Who was Liu?In a statement, Liu's colleagues describe him as an outstanding researcher who "was on the verge of making very significant findings" towards understanding the cellular mechanisms of Covid-19 infection.
They mourned Liu's passing and pledged to complete his research "in an effort to pay homage to his scientific excellence".
Liu, a native of China, earned his Bachelor's degree and PhD in computer science in Singapore before conducting research in the US.
According to his online CV, he had collaborated with biologists and clinicians to study human immunity.
What are the conspiracy theorists saying?"Oh my god," a user on the Chinese social media platform Weibo wrote. "This seems like coming straight out of Mission Impossible. Perhaps he found out that the virus originated from an American lab."
Many similar comments suggested Liu was killed because he was supposedly about to unravel the mystery of coronavirus' origin.
Chinese officials and state media had previously promoted a baseless claim that the virus originated in the US and was brought to Wuhan by American soldiers.
Some Weibo users said the case "seems too coincidental".
"A very unusual case. There are likely secrets hidden in the dark," one remark read.
Many Weibo comments suggested Liu's Chinese background may have put him at risk in the US, though no evidence has emerged that Liu was targeted because of his ethnicity.
Global Times, a website affiliated with the Chinese state media, published an article various speculation surrounding Liu's death.
On Twitter, meanwhile, some have speculated on unfounded claims that the Chinese government may have played a role in the case.
Amid the Covid-19 pandemic, a variety of conspiracy theories about the virus and its origins continue to be spread on online platforms. Some unverified claims have been promoted by politicians and media outlets in both the US and China.