The man charged with killing seven people in a shooting on an Independence Day parade in Chicago has confessed to police that he fired on the crowd.

Robert E Crimo III faces seven counts of murder over the shooting in the Chicago suburb of Highland Park, Illinois, and news of the confession – which was reported by the Lake County State’s attorney – came as he appeared in court.

Police also revealed that Crimo had considered a second shooting in Madison, Wisconsin, but – despite having 60 rounds of ammunition remaining – he had not planned for this, so did not go ahead.

Crimo, 21, had planned the Chicago attack for several weeks, police said, and is alleged to have climbed a fire escape ladder onto the roof of a business before firing 83 rounds at the crowd with a high-powered assault rifle.

The state’s attorney said that Crimo admitted dressing up as a woman to blend in with the crowd, climbing on the roof, looking through his sights and firing down the street with a Smith and Wesson M&P 15 rifle.

If convicted, he will face a mandatory sentence of life in prison without the possibility of parole, Illinois state attorney Eric Reinhart said.

Speaking on Tuesday, he added that “dozens of more charges” are expected to follow.

In court, the judge agreed with the prosecutor’s request that Crimo be held without bail.

Among those killed in Monday’s attack were Nicholas Toledo, a grandfather from Mexico in his 70s, and Jacki Sundheim, a teacher at a nearby synagogue.

It came just weeks after 19 children and two teachers were shot dead at an elementary school in Uvalde, Texas, and 10 people were shot dead at a grocery store in Buffalo, New York.

There have been 15 shootings this year in which four or more people have been killed, according to a database maintained by The Associated Press, USA Today, and Northeastern University.