The attack on demonstrators in Boulder, Colo., marching in support of Israeli hostages would have been disturbing to Jewish people across the country even if it was the only such event of its kind. The suspect told investigators after his arrest that he had been planning the attack for a year, according to court documents. Eight people were hospitalized.

For many, the connections to other recent outbursts of violence were impossible to miss. The attack in Boulder came less than two weeks after two Israeli Embassy employees were shot and killed as they left a reception at a Jewish museum in Washington. A month earlier, an arsonist set fire to the Pennsylvania governor’s mansion on the first night of Passover while Gov. Josh Shapiro, who is Jewish, slept upstairs with his family.

“What we’ve seen these last few months is a shocking pattern of anti-Israel sentiment manifesting itself in antisemitic violence,” said Halie Soifer, chief executive of the Jewish Democratic Council of America. “With each incident there’s a further shattering of our sense of security.”

In Colorado and Washington, authorities said, the suspects shouted “Free Palestine” on the scene. In Pennsylvania, the arsonist later said he set the fire as a response to Israeli attacks on Palestinians.

Ms. Soifer pointed out that the Molotov cocktails used by the attacker in Boulder were strikingly similar to the incendiary devices used by Cody Balmer, the accused arsonist in Pennsylvania.