The authorities carried out a search of Mr. Martin's home on Friday night.
For hours on Friday afternoon, a chaotic scene took place in Aurora, a town of 200,000, as fire trucks, ambulances and police brigade cars invaded the area of homes and warehouses.
Concerned parents waited while schools – including one not far from the warehouse – ordered students to stay there for more than an hour. The agents searched the blocks around the warehouse. Relief officials waited to wait to bring more patients to hospitals. And officials from the coroner's office were called.
"It is unfortunate that mass fire like this has become commonplace in our country," said Aurora mayor Richard C. Irvin. "It's unfortunate that a cold, heartless delinquent is selfish enough to think he has the right to lead an innocent life."
In the evening, political leaders from across the country began to publish press releases and post messages on social media, offering sympathy and support.
The White House said President Trump had been informed of the situation and was monitoring it. The new governor of Illinois, J. B. Pritzker, went to Aurora and paid tribute to the police who entered the interior during the shooting.
"You rushed to danger and, in doing so, saved countless lives," said Pritzker.
Rosalee Andrada, 54, lives in a house on Cleveland Avenue, just steps from the building where the shooting took place. She had just been taking her medicine every day when she heard staccato noises coming from outside.
"I thought it was just ice that was cracking," she said.
But her husband, Jose, and his husky, Rocky, were rushed into the cellar. She said she heard five shots in total.