At least three people were killed, and nearly 800,000 customers lost power as severe storms swept across the US Midwest on Tuesday, according to authorities.

Two deaths occurred in the state of Indiana; one person died in a suspected weather-related house explosion in Portage, while a boy, 4, was killed when a tree fell on his home in Jennings County, according to NBC News.

“Contributions are probably wind and the storms that came through this county earlier today,” Jennings County Chief Sheriff’s Deputy Cody Low said about the child’s death.

Another person died in Roseville, Ohio, after emergency crews were unable to reach the victim, said Gov. Mike DeWine. A state of emergency was declared following widespread flooding, with flash flood warnings affecting 16 million people Wednesday.

In Portage, severe weather and trees blocking roads delayed firefighters responding to the house fire. Police Capt. Rob Maynard described the storms as “probably the most severe weather event that any of us here can remember.”

Sixteen inmates at a prison in Grafton, Ohio, were taken to hospitals after lightning struck as they returned from dinner; one was airlifted, said authorities.

The National Weather Service confirmed two tornadoes near Chicago, both producing EF1-level damage. Nearly 800,000 customers in Illinois, Indiana, Ohio and Kentucky remained without power Wednesday morning. Most of them in Ohio and Indiana.

Utility provider AEP said the Columbus, Ohio, area experienced gusts of more than 80 mph (129 km/h) and trees falling on power lines. It said it had more than 2,000 workers trying to restore electricity, including crews from other states.