Belarusian security forces shot dead a 31-year-old man, identified by the pro-democracy opposition as an IT worker with US company EPAM Systems, during a raid on Tuesday (Sep 28) at an apartment block in the capital Minsk.
The man had opened fire against the security forces, one of whom also died, the KGB security service said in a statement. It also said his wife had been arrested.
The Investigative Committee of Belarus, which investigates major crimes, said a 31-year-old man was "liquidated with return fire" after resisting law enforcement officers.
Reuters could not independently verify the statements or footage of the incident that was aired on Belarusian state television.
EPAM declined immediate comment.
Forces loyal to President Alexander Lukashenko unleashed a violent crackdown on protests following a disputed election last year, including searches of apartment blocks where they believed protesters were hiding.
In power since 1994, Lukashenko has defied Western sanctions and opposition calls to step down and described the protesters as criminals bent on violent uprising.
The IT industry - mainly housed in a state-run technology park on the outskirts of Minsk - was a driving force behind the protests. EPAM's Belarusian founder was a signatory to an open letter calling for the release of prisoners and new elections.
Local media and a senior adviser to exiled opposition leader Sviatlana Tsikhanouskaya identified the man killed as Andrei Zeltser, an IT worker with EPAM. A LinkedIn page of a man with the same name states he has worked at EPAM since 2016.
"He was also reportedly a US citizen. According to his friends, he supported democracy movement in Belarus. His wife is detained. Mother is shocked," Franak Viačorka, Tsikhanouskaya's adviser, wrote on Twitter.
"I have only one comment: violence must stop! The regime hopes that violence can split society and threaten opponents. But violence cannot be the solution to the crisis. Enough victims. The rule of law must return to the country."
Lukashenko's spokeswoman said the death of the KGB officer would "not remain unpunished".
The US embassy did not provide immediate comment.
The official state news agency Belta reported the man was associated with the opposition movement, citing a lawmaker. The KGB did not identify the man by name or profession but said he was a "terrorist" - language it uses to describe protesters.
"I can't say or think anything. I'm shocked, I don't understand what's going on," Belarusian media outlet Nasha Niva quoted Zeltser's mother-in-law as saying. "Andrei is a very good person, a wonderful son-in-law."
"Do I consider Andrei a terrorist? Of course not," she said.
Footage shown on state television showed plainclothes officers breaking down an apartment door and a man firing at them as they entered.
"In response to the lawful demands of law enforcement officers, a 31-year-old resident of Minsk refused to open the door of the apartment and was locked inside it. For the subsequent so-called 'hype', he was filming," the Investigative Committee of Belarus said in a statement.
"Given the nature of the violence used, armed resistance from a 31-year-old man, he was liquidated with return fire."