A Portland activist who said a police flash-bang grenade struck the back of his head while he demonstrated against a right-wing rally plans to sue the city, new documents show.

The munition slammed into Aaron Anthony Cantu, 35, lodging itself into his bicycle helmet and splitting open his skull, he said.

Cantu, who detailed the incident and its bloody aftermath in an interview with The Oregonian/OregonLive last month, went to the hospital. Photos of his mangled scalp and ruined headgear went viral.

"Mr. Cantu had suffered a traumatic brain injury, likely one that would have been fatal had not he been wearing a bike helmet," reads a Sept. 17 letter sent to the city and signed by civil rights attorneys Juan Chavez and Crystal Maloney.

A so-called "less lethal" munition, flash-bangs are meant to deliver jarring blasts that disperse crowds. But they can be fatal.

A ProPublica investigation found that 50 Americans were seriously injured, maimed or killed by flash-bangs between 2000 and 2015.

Cantu's tort claim notice, first reported Wednesday by Willamette Week, is the second filed by an individual who says they were injured or maimed by Portland police while protesting an Aug. 4 Patriot Prayer rally at Tom McCall Waterfront Park.

Last week, a lawyer for Michelle Fawcett, 53, wrote her client suffered third-degree chemical burns on her arms and chest in August during an "unprovoked attack on her by members of the Portland Police Bureau."

Cantu's claim, which is a formal notice of his plans to sue the city, largely recounts what he said in the Aug. 10 interview attended by Chavez and Maloney. At that time, he asked The Oregonian/OregonLive not to disclose his full name for safety reasons.

An industrial mechanic and self-described anti-fascist activist, Cantu had gathered with dozens of other demonstrators near Southwest Columbia Street and Naito Parkway when police ordered the group to disperse.

Cantu, a Southeast Portland resident, said he did not recall hearing the police order. But he and people near him heard deafening explosions and turned to retreat, he said.

He was a half block from Naito when the flash-bang slammed into the back of his head.

"I didn't know if my brains had spilled from my head or if I was about to bleed out in the street," he said. "I thought I was going to die."

He was treated at the scene by several volunteer street medics and eventually taken to the hospital, where he spent the next 24 hours. The injury caused hemorrhaging, and he had a tube inserted in his skull to drain the fluids, he said.

According to his legal notice, the police reaction violated Cantu's First, Fourth and Fourteenth amendment rights, and his alleged injuries could constitute "negligence, battery, [and] intentional infliction of emotional distress."

A spokeswoman for Mayor Ted Wheeler said the office does not comment on pending litigation. Sgt. Chris Burley, a Portland police spokesman, said the bureau's protest response is under review.

-- Shane Dixon Kavanaugh

skavanaugh@oregonian.com

503-294-7632 || @shanedkavanaugh