(CNN) -- The pilot and co-pilot of a small cargo jet were killed when their aircraft crashed short of the runway while landing at an airport in suburban Chicago, Illinois, local authorities said.

The Learjet 35A was on its final approach into Chicago Executive Airport in Wheeling, Illinois, when it went down about 1:30 p.m., the Federal Aviation Administration reported. The plane crashed into a forest preserve about a mile south of the runway and ended up in a river, said Jay Reardon, the head of a multi-agency firefighting task force that responded to the crash.

The two-man crew was found dead at the scene, Reardon said. No emergency was declared before the crash.

"To our knowledge, everything appeared normal up until the aircraft was reported down by the tower at Chicago Executive Airport," Reardon said.

The National Transportation Safety Board has begun its investigation of the crash. There was no immediate indication of the cause, NTSB senior investigator Pam Sullivan said.

"It's way too early to even start narrowing down what could or could not happen," she said.

The flight was operated by Royal Air Cargo, a Waterford, Michigan-based carrier that operates a fleet of small jets to deliver "time-sensitive" freight, airport spokesman Michael Standard said. Reardon said the jet appeared to be empty when it went down.

The downed plane was en route to Atlanta, Georgia, from Pontiac, Michigan, with a stop in Chicago, according to private Web sites that track air travel. There was no immediate comment from the company.