CHICAGO — A gunman involved in a shooting that wounded 13 people at a South Side park on Thursday night was armed with an assault-style rifle equipped with a high-capacity magazine, the police said on Friday.

“A military-grade weapon on the streets of Chicago is simply unacceptable,” Garry McCarthy, the Chicago police superintendent, said at a news conference, where he issued another of his frequent calls for tighter state and federal gun laws.

Among the wounded was a 3-year-old boy, who was struck in the face but was expected to survive. The boy and two other people, also expected to survive, were hospitalized in critical condition. Mr. McCarthy said it was a miracle that no one was killed.

The police said that no arrests had been made and that investigators were uncertain about the intended target, the motive, or even the number of people who fired weapons. There may have been only one gunman, Mr. McCarthy said, or as many as three.