The authorities said they were investigating an attack in Boulder, Colo., on Sunday as an act of terrorism, after a man used a “makeshift flamethrower” to attack demonstrators honoring Israeli hostages in Gaza.

Six people were hospitalized with burns and other injuries, and two of them were in serious condition, officials said.

Witnesses said the man threw an incendiary device into the crowd in a downtown pedestrian mall, according to the authorities. The suspect, identified as Mohamed Sabry Soliman of Colorado Springs, yelled “Free Palestine” during the attack, the witnesses said. Mr. Soliman, 45, had not been charged as of late Sunday night.

The attack may intensify deep unease in the Jewish community in the United States. In recent months, two Israeli embassy aides were murdered in Washington, and a man set fire to the residence of Gov. Josh Shapiro of Pennsylvania, who is Jewish.

Here’s what we know:

The attack targeted a peaceful event.

The victims, who range in age from 67 to 88, were participating in a weekly event called Run for Their Lives that is held in cities around the world, including Boulder. It is designed to call attention to the Israeli hostages taken by Hamas in the Oct. 7 terrorist attacks.

The police began to receive calls about an emergency on the Pearl Street Pedestrian mall near Boulder’s county courthouse at 1:26 p.m., Boulder’s police chief, Steve Redfearn, said at a news conference on Sunday evening.