Michael Reinoehl was under investigation into the attack that left Aaron Danielson dead on Saturday. According to the New York Times, Mr. Reinoehl was killed Thursday evening following a meeting with law enforcement.
Some of these activists believe that the use of violence against those they consider extreme right is legitimate.
Mr Reinoehl was reportedly killed as police intervened to arrest him in Lacey, southwest of Seattle.
Portland Police issued an arrest warrant for him on Thursday, although federal forces are attempting to detain him.
Mr. Danielson’s shooting took place after violent clashes erupted between Trump supporters and opponents in the city.
A large convoy of vehicles carrying Trump supporters was passing through the city when they encountered counter-protesters.
Video from the scene showed the two sides fighting with fists, paintball guns and a bear mace.
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Mr Reinoehl said he “had no choice” but to use his gun during the confrontation.
He added, “I could have sat there and watched them kill one of my colored friends. But I wasn’t going to do that.
Police did not say that the shooting victim, Mr. Danielson, carried a gun or any other lethal weapon.
Protests erupted in Portland, as across America, after the death of George Floyd while in police custody in May.
While protests have calmed down across much of the country, they have continued until now for an almost overnight base in Portland.
Some of the protesters, many dressed in black, were involved in violent clashes with police.
Mr Reinoehl is said to have participated frequently in these demonstrations.
According to the New York Times, he had previously warned against violence.
Posting on Instagram, he reportedly said: “I am ready to fight for my siblings!
“Even though some of them are too ignorant to understand what antifa really represents.
“We don’t want violence, but neither will we run away!”