Hennepin County Attorney Mike Freeman said Wednesday he won't file charges against Brooklyn Center police in the fatal shooting of a 21-year-old man last summer because the officers had a "reasonable fear" that they were in danger.

In a news release, Freeman said the two officers who fired reasonably believed they and Kobe Dimock-Heisler's grandmother were in "danger of death or great bodily harm" when they fired six shots at him and killed him.

"Both officers saw Mr. Dimock-Heisler attempting to stab Officer [Joseph] Vu with a knife," Freeman said. Four officers at the scene tried to subdue him "with Tasers to no avail" and were justified in resorting to deadly force.

Dimock-Heisler's parents, together with a citizens coalition formed to combat police violence, held a news conference Wednesday afternoon in front of the Government Center in Minneapolis to demand an independent prosecutor and investigation of the incident.

His mother, Amity Dimock, said police escalated the situation and "ended up putting my son down like an animal."

Alongside a banner that read "Jail Killer KKKops," several activists called for Freeman to resign.

"Amid growing demands for Freeman's resignation, the people of Hennepin County need someone who's willing to prosecute police, not act as their defense attorneys. Last September, Brooklyn Center police entered an already calm situation and escalated it into a deadly nightmare for Kobe and his loved ones," said the Twin Cities Coalition 4 Justice 4 Jamar, which was formed in the wake of the fatal shooting of Jamar Clark by Minneapolis police officers in 2015.

The shooting of Dimock-Heisler on Aug. 31 began when he went to a fast-food restaurant with his grandfather. Dimock-Heisler, who suffered from mental illness and was on the autism spectrum, became angry and yelled at the employees when they got the order wrong. He then became angry at his grandfather, who had told him to stop yelling, Freeman's office said.

Their argument continued when they got home, where Dimock-Heisler grabbed a small, serrated knife and a hammer and told his grandfather to apologize, according to the prosecutor who cited the Minnesota Bureau of Criminal Apprehension investigation. The grandfather, who was scared, managed to get away, slip into a back bedroom and call 911.

About 4:20 p.m., officers Cody Turner and Brandon Akers responded in separate cars to the home in the 5900 block of Halifax Ave. N.

As they were driving to the house, Akers pulled up a report of another incident at that address six months earlier and Turner remembered he had been there when Dimock-Heisler had stabbed himself in the stomach.

When they got there, the grandfather told them the situation had calmed down.

However, the two officers, along with officers Steven Holt and Vu, who were in a separate car, explained that they were required to go into the house to assess the situation.

Vu found no weapons on Dimock-Heisler and had him sit in a chair in the living room, while his grandmother sat in a couch, facing him. She also gave police the hammer and knife.

Vu calmly spoke with Dimock-Heisler and got him to pull up his shirt and reveal where he had cut himself. Dimock-Heisler said he had previously been on a 30-day mental health commitment and didn't want to return.

But after putting his face in his hands and crying, Dimock-Heisler suddenly sprang up and started to run toward his grandmother, Freeman said. Holt and Vu moved to stop him and the three knocked over another couch in the room.

Turner and Akers, who were outside with the grandfather, heard the commotion and charged inside. Turner, Holt and Akers fired their Tasers, but Dimock-Heisler was able to retrieve a hidden knife from the couch and tried to stab Vu, who was hanging onto his lower legs.

Freeman said Turner and Akers fired their semi­automatic handguns three times each, striking Dimock-Heisler in his chest and neck. Video from the officers' body-worn cameras showed a "very quick and chaotic scene," Freeman noted.

"We are saddened by the death of Mr. Dimock-Heisler and we have extended our sympathies to his grandfather and grandmother who have raised him since age six," Freeman said.

Star Tribune staff writer Andy Mannix contributed to this report.

Correction: Previous versions of this article misstated the last name of Amity Dimock.