The Illinois State Police said on Thursday that the woman who drove her Jeep through an after-school center in Chatham, Ill., earlier this week could have experienced a medical episode before the crash, which killed three children and a teenager.

The driver, a 44-year-old Chatham resident, tested negative for alcohol and controlled substances in the aftermath, authorities said.

“Some evidence has been developed indicating the possibility of a medical emergency leading up to the crash,” Brendan F. Kelly, the director of the Illinois State Police, said at a news conference on Thursday. “However, the investigation of this information and other evidence has not yet concluded, and will continue until all leads and research have been exhausted.”