The suspect in the Boulder, Colo., attack on supporters of Israeli hostages in Gaza is an Egyptian citizen who is in the United States on an expired tourist visa, the Department of Homeland Security said on Monday, as more details emerged about the episode that left eight people hospitalized with burns and other injuries.

The suspect, Mohamed Sabry Soliman, entered the United States from Egypt in August 2022 and stayed illegally after the visa expired in February 2023, said Tricia McLaughlin, a spokeswoman for the Department of Homeland Security.

“The Colorado Terrorist attack suspect, Mohamed Soliman, is illegally in our country,” Ms. McLaughlin said in a post on social media. She added that he had filed for asylum in September 2022, but gave no additional details.

The attack, which authorities were investigating as an act of terrorism, occurred on Sunday when a man used a “makeshift flamethrower” to attack people who were marching peacefully in support of Israeli hostages being held in Gaza, officials said. Mr. Soliman, 45, was taken into custody after witnesses identified him as the assailant, and was booked on multiple charges in the Boulder County Jail.

The charges included first-degree murder, but the authorities did not immediately confirm whether any of those injured in the attack had died. Two of those injured were in serious condition on Sunday, officials said.

A U.S. official said that the National Counterterrorism Center had shared information with federal law enforcement officials on Sunday that showed no preliminary derogatory information about Mr. Soliman.

Adam Goldman contributed reporting.