Emergency crews responded around 10:15 p.m. to reports of a structure fire at a home in the 700-block of East Grant Highway, also known as US Highway 20, police said.
A next-door neighbor said she heard the raging fire and called 911.
"She heard an explosion. She heard three pops, I think, is the way she described it," Marengo Fire Chief Bob Bradbury said.
The officers arrived first. Officer Andrew Kjellgern said he saw smoke and flames when he pulled up to the house. He spotted a car in the driveway but didn't see anyone outside, so he feared someone was trapped inside.
Kjellgern said he and other officers found the door locked, so they kicked it in and rushed inside.
"I could see there was a fair amount of smoke on the first level. And I could see the glow of the fire from the stairway," he said.
Once inside, Kjellgern called out. A man responded, crying for help. The officer followed the sound of the man's voice and found him on the floor. He appeared to be injured and wasn't able to get up.
"He looked up at me with kind of half-open eyes and asked for help," Kjellgern said.
He and his sergeant picked up the man and carried him outside. He was airlifted to Loyola University Medical Center. At last word, he was in serious condition.
Kjellgern said he's gotten a few calls about his bravery, but downplays what he did.
"I'm just doing my job. It's what we're out there to do; just make sure people are safe and help people out," he said.
"The heat was coming down the stairs, but they still managed to get him out. They did a great job," Bradbury said.
Meanwhile, firefighters stopped the flames from spreading to any nearby homes.
Bradbury said fire crews were lucky the police officers were brave enough to enter the burning duplex. He said the victim was lucky too.
"We were glad to have them here. So was he," Bradbury said.
US Highway 20 was briefly closed for an investigation. The state fire marshal went to the scene Tuesday to determine how the blaze started.