(CNN) A man under federal investigation in a potential domestic terrorism case was shot and killed Tuesday during an FBI operation to arrest him, people familiar with the investigation said.

The suspect, who hasn't been identified by authorities, was the subject of a months-long investigation into alleged white supremacist activity, and recently had made threats the FBI took seriously, the sources said.

Among the threats was one to carry out an attack on a hospital in the Kansas City, Missouri, area, where he lived, in part because of the belief that patients may be treated there for the Covid-19 virus.

It's not clear why the FBI agents opened fire during the attempted arrest of the suspect during what was supposed to be a controlled operation in Belton, Missouri. Typically in sting operations such as the one Tuesday, investigators use undercover agents or informants to gather direct evidence to prove that a suspect was actively planning to carry out an attack.

The FBI acknowledged the shooting and the man's death, but declined to release additional information.

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