A house party on Chicago’s South Side was meant to remember a man who was fatally shot during a carjacking in April, the authorities said. But the memorial itself erupted into violence early Sunday after at least two gunmen opened fire, wounding 13 people, four of them critically.

The shooting took place just after 12:30 a.m. in the Englewood neighborhood on what was the birthday of the man killed in April, said Tom Ahern, a spokesman for the Chicago Police Department.

About 70 people, including friends and family of the man, were in the two-story house at the time, he said.

The motive for Sunday’s shooting was not immediately clear.

Fred Waller, chief of patrol for the Chicago Police Department, said at a news conference that the authorities were still investigating any connection between the April killing and Sunday’s shooting. Two people were being questioned in connection with Sunday’s shooting, Chief Waller said.

He said he did not believe the shooting was gang-related, and that it was likely an “isolated incident” stemming from a “personal dispute.”