The McHenry County coroner idenfited the victims as Hugh Scott Clark, 65, of Lake Forest, and Paul Sanfilippo, 82, of Grayslake.
The single-engine Beech 35 aircraft plunged into a marsh-like, muddy pit around 3:15 p.m. A passerby dialed 911.
"It was really loud, really loud," said witness Katie Garcia. "It sounded just like something blew up."
The crash site is close to Lake in the Hills Airport near Pyott and Virginia Roads in Crystal Lake. The terrain was so difficult first responders had to trudge to the scene on foot. Police say a bulldozer arrived, plowed a road, and then responders used all-terrain vehicles.
"Access to the scene was very difficult," said Deputy Chief Gene Lowery. "Unfortunately, when the first responders got to the scene they discovered that the plane had been seriously damaged and that both occupants were deceased."
Witnesses tell police it looked like the plane was trying to return to the airport.
"As it began its bank left, witnesses advise that they heard the plane decelerate and then begin to lose altitude and then didn't visually confirm it, but they heard a loud noise indicating that the plane had impacted," said Lowery.
The FAA and the National Transportation Safety Board are investigating the crash.
"We just have to check the airplane, go through it and make sure it's OK," said Al Pattenaude, FAA investigator. "The airplane is secure, and we'll be back in the morning."