Immigration authorities have arrested the wife and children of the man suspected of carrying out Sunday's attack in Boulder, Colorado, US Homeland Security Secretary Kristi Noem said.

Mohamed Sabry Soliman, 45, has been charged with attempted murder, assault and possession of an incendiary device after a dozen people were injured at a march calling for the release of Israeli hostages.

"We are investigating to what extent his family knew about this heinous attack, if they had knowledge of it, or if they provided support to it," Noem posted on X.

Officials say the suspect shouted "Free Palestine" as he threw two petrol bombs into the crowd. He also faces a federal hate crime charge.

The US homeland security secretary posted a video about how the defendant's wife and five children had been taken into custody.

But she did not say if there were plans to deport the family members or what would happen to them. Mr Soliman's daughter reportedly graduated from high school recently.

Homeland security is responsible for enforcing immigration laws, but the FBI, justice department and local police typically carry out investigations into possible criminal activity.

Twelve people, including four men and four women between the ages of 52 and 88, were taken to hospital, with injuries ranging from minor to serious.

The eldest of the victims is a Holocaust survivor, Rabbi Israel Wilhelm, the Chabad director at the University of Colorado Boulder, told the BBC's US partner CBS News.

In an affidavit, police said Mr Soliman had admitted carrying out the attack and that he had been planning it for a year.

What we know about the attack in Colorado

Homeland security officials have said that Mr Soliman arrived in the US on a tourist visa in August 2022. That visa expired the following year, but he made an asylum claim in September 2022.

According to police documents, the suspect told officials that he "never talked to his wife or his family" about his plans, and that he had left a phone in a desk drawer with messages to his wife and children. His wife turned the phone in to authorities.

One of Mr Soliman's daughters was recently awarded a scholarship by a local newspaper in Colorado Springs.

"Coming to the USA has fundamentally changed me," she wrote in the scholarship application.

"I learned to work under pressure and improve rapidly in a very short amount of time. Most importantly, I came to appreciate that family is the unchanging support."

The newspaper reported she was interested in studying medicine.

A profile in the Gazette newspaper noted she "was born in Egypt but lived in Kuwait for 14 years" and relocated to the US two years ago.

The newspaper also reported: "When she was young her father underwent a difficult surgery that restored his ability to walk."

According to the FBI, Mr Soliman said he "was waiting until after his daughter graduated to conduct the attack." His daughter's high school graduation ceremony was last week.

Soliman appeared in court on Monday via a video feed from the Boulder County Jail for less than five minutes, standing and wearing an orange jumpsuit.

He answered "yes" to some procedural questions from the judge, but otherwise did not speak.

He is next due in court for the formal filing of charges and a bail hearing on Thursday. (BBC)