It happens every week in Boulder, Colorado.
A group of volunteers from the Run for Their Lives organisation silently march through the streets to raise awareness of the hostages still held in Gaza.
About 20 or 30 had turned up this Sunday, stopping at the courthouse where they usually read the names of those hostages.
"There was somebody there that I didn't even notice," participant Ed Victor told CBS News.
"Although he was making a lot of noise, but I'm just focused on my job of being quiet and getting lined up.
"And from my point of view, all of a sudden, I felt the heat."
Lynn Segal, 72, was also among the group gathered when a "rope of fire" shot in front of her and then "two big flares".
She said the scene at the popular Pearl Street pedestrian mall, a four-block area in downtown Boulder, quickly turned chaotic as people worked to find water to put out flames and find help.
"There were people who were burning. I wanted to help but I didn't want to be associated with the perpetrator," said Ms Segal, who said she was wearing a pro-Palestinian T-shirt.
The FBI said the suspect allegedly shouted "Free Palestine" while using a makeshift flamethrower at a crowd of people.
FBI leaders in Washington said they were treating the Boulder attack as an act of terrorism.
The Justice Department said the attack was a "needless act of violence, which follows recent attacks against Jewish Americans".
However, local police in Boulder were cautious about describing the incident as a terror attack.
"It would be irresponsible for me to speculate on motive this early on," Boulder Police Department Chief Steve Redfearn said.
What happened?
Chief Redfearn said authorities received a call at 1:26pm on Sunday, local time, that indicated a man armed with a weapon was setting people on fire.
Eight people, aged between 52 and 88 years old, were injured.
He said the injuries were consistent with reports of individuals being set on fire.
They were taken to the Boulder Community Hospital with injuries ranging in severity from "very serious" to "minor", and some were later transported to other hospitals.
At least one of them was in a critical condition, authorities said.
The Boulder Police Department evacuated the pedestrian mall area.
Law enforcement officers with a police dog walked through the streets, securing the area and examining a "vehicle of interest".
Brooke Coffman, a 19-year-old at the University of Colorado who witnessed the incident, said she saw four women lying or sitting on the ground with burns on their legs.
She said one of them appeared to have been badly burned on most of her body and someone had wrapped her in a flag.
Ms Coffman described seeing a man who she presumed to be the attacker standing in the courtyard, shirtless, holding a glass bottle of clear liquid and shouting.
"Everybody is yelling, 'Get water, get water,'" Ms Coffman said.
Who is the alleged suspect arrested?
The FBI has identified the suspect as 45-year-old Mohamed Sabry Soliman.
He was taken into custody at the scene and transported to hospital, but authorities did not elaborate on the nature of his injuries.
Officials did not immediately announce any charges but said they expected to hold him "fully accountable".
Chief Redfearn said he did not believe anyone else was involved in the incident.
Mark Michalek, the FBI special agent in charge of the Denver field office, said the federal law enforcement agency was investigating the attack.
"As a result of these preliminary facts, it is clear that this is a targeted act of violence and the FBI is investigating this as an act of terrorism," he said.
The incident came just weeks after the fatal shooting of two Israeli embassy employees in Washington DC.
In that case, authorities allege a Chicago-born man opened fire on a group of people leaving an event hosted by the American Jewish Committee, an advocacy group that fights anti-semitism and supports Israel.
FBI describe attack as terrorism but local police urge caution
FBI director Kash Patel described the latest incident as a "targeted terror attack".
His words were echoed by the bureau's deputy director, Dan Bongino, in a post on X.
"This act of terror is being investigated as an act of ideologically motivated violence based on the early information, the evidence, and witness accounts," Mr Bongino said.
"We will speak clearly on these incidents when the facts warrant it."
However, Chief Redfearn called for caution, saying it would be irresponsible for him to speculate on motive so soon after the incident.
"We are not calling it a terror attack at this moment," Mr Redfearn said.
"We've got dozens and dozens of people here working through this to work out exactly what happened."
Colorado Attorney-General Phil Weiser, meanwhile, said the incident appeared to be "a hate crime".
"My thoughts are with those injured and impacted by today's attack against a group that meets weekly on Boulder's Pearl Street Mall to call for the release of the hostages in Gaza," Mr Weiser said.
"People may have differing views about world events and the Israeli-Hamas conflict, but violence is never the answer to settling differences.
"Hate has no place in Colorado. We all have the right to peaceably assemble and the freedom to speak our views."
Hamas-led militants stormed southern Israel on October 7, 2023, killing some 1,200 people, mostly civilians, and abducting 251 hostages. More than half the hostages have been released in ceasefires or other deals. Israel has rescued eight and recovered dozens of bodies.
Israel's ensuing military campaign has killed more than 54,000 Palestinians, mostly women and children, according to the Gaza Health Ministry, which does not say how many of the dead are civilians or combatants.
The offensive has destroyed vast areas of Gaza and displaced about 90 per cent of its population of roughly 2 million Palestinians, according to the United Nations estimates.
ABC/wires
