Mohamed Sabry Soliman told the police that he had tried to disguise himself as a gardener on Sunday afternoon when he headed toward a group that was walking in downtown Boulder, Colo., to remember the hostages being held in Gaza, the authorities said.

Mr. Soliman, a 45-year-old born in Egypt, carried flowers he had bought from a Home Depot store, according to a Boulder police detective. He wore an orange vest. And he had strapped on a backpack sprayer, the kind that gardeners often use to apply fertilizer or pesticide.

But the sprayer was full of gasoline.

The fiery weekend terror attack that the authorities say Mr. Soliman soon carried out — in a plot he said he had hatched himself — injured 12 people, who were burned by two homemade Molotov cocktails that the authorities say he threw into the crowd. Mr. Soliman yelled “Free Palestine” during the attacks, the authorities said, and later told the police he “wanted them all to die” because he believed the demonstrators were “Zionists” supporting the occupation of Palestine.

Before Sunday, Mr. Soliman appeared to have lived a prosaic life in Colorado Springs, where he drove for a ride share service and was raising five children with his wife in a worn stucco apartment amid the dry, windy suburban stretch east of town. He told the police he had assembled his dangerous arsenal of explosives from everyday household goods.

But the assault resonated far beyond Boulder. It came roughly two weeks after another supporter of the Palestinian cause killed two Israeli embassy workers in Washington, D.C., sending fresh waves of fear through Jewish communities around the world whose members were left wondering if anywhere was safe for them as Israel’s war in Gaza grinded on.

Mr. Soliman was arrested minutes after the attack and was being held on a $10 million bond. Police officers found him on a patch of grass near the Boulder courthouse, shirtless and screaming at the crowd, holding two Molotov cocktails. At least 14 other Molotov cocktails were found near him in a black plastic container.