Two teenagers were arrested on Sunday in connection with a targeted shooting in southern Illinois that left five members of the same family dead and two other relatives seriously injured, the Illinois State Police said at a news conference on Sunday.

The suspects, ages 15 and 16, had not been formally charged as of Sunday evening, and police investigators declined to discuss a possible motive for the shootings.

The attacks occurred at three locations in East St. Louis. The authorities said that they believed at least one of the suspects was related to the slain family members.

“These alleged aspects of horrific violence, taking this number of lives here in this community — it’s terrible, it’s evil, but it will not keep this city down,” Brendan Kelly, the director of the Illinois State Police, said at the news conference.

East St. Louis has long struggled with violent crime, but Mr. Kelly said that law enforcement officers had been working for years to reduce the city’s crime rates.

Mr. Kelly said that the suspects were arrested on Sunday morning after dispatchers received a call “related to investigating human remains.”

The police identified the people killed as Cherie L. May, 49; Devin D. May, 24; Patricia A. May, 74; Quentin L. Thompson, 21; and Shania W. Thompson, 25.

Officials did not say when the victims were killed or exactly how they were related to one another or to the suspects.