ROSELAWN, Ind. -- The last transmission from the cockpit of American Eagle Flight 4184 was a simple, "Thank you," federal investigators said yesterday as the hunt began for the cause of the Monday night crash that killed all 68 people on the plane.
An examination of radio transmissions between the plane and controllers at the Chicago Air Route Traffic Control Center contained no words of distress or any verbal clues as to what caused the twin-engine turboprop to plummet at least 8,000 feet into a northwest Indiana farm field during a driving rainstorm.
Whatever went wrong apparently happened so fast that the crew did not have time to declare an emergency.
In a news conference last night in Merrillville, Ind., National TC Transportation Safety Board Chairman Jim Hill said high-resolution Doppler radar images from the National Weather Service in Joliet, Ill., taken at the time of the accident indicated "nothing remarkable" in the weather conditions, despite the wind and rain and reports in the area of wind shears -- sudden shifts in wind direction that can be treacherous for aircraft.
Still, weather remains a focus of the safety board's investigation into the crash of Flight 4184, which was in a holding pattern awaiting clearance to land at Chicago's O'Hare International Airport after flying from Indianapolis.
Mr. Hill said the National Weather Service broadcast two aviation advisories while the plane was in the air, one for light to moderate turbulence and another for light to moderate icing.
Investigators will check the plane's cockpit voice recorder, one of two recording devices recovered after the crash, to see if the crew acknowledged receiving the advisories, he said.
If there is no acknowledgment, that could mean that the crew was unaware of the conditions and did not take precautionary steps, such as activating anti-icing equipment.
One aviation-safety expert who spoke on condition of anonymity speculated that if the plane had accumulated ice as it circled in its holding pattern for 30 minutes before starting to descend and the build-up created aerodynamic changes that weren't detected by the pilot, heavy buffeting by strong and changing winds could have caused a stall and loss of control.
The cockpit voice recorder and the flight data recorder, which was damaged on the outside by the impact of the plane but was reported to be intact internally, were taken to the safety board's laboratory in Washington, Mr. Hill said.
He said that, after arriving over the Chicago metropolitan area, Flight 4184 went into a holding pattern at 3:24 p.m. that lasted 32 minutes; then the crew received clearance from the Chicago air traffic control center in Aurora, Ill., to descend to 8,000 feet from 10,000 feet.
Seconds later, the control center advised the pilot that he would be in a holding pattern for a further 10 minutes.
"Thank you," he replied.
"That was the last transmission," Mr. Hill said.
With daybreak and clear weather yesterday, the crash scene became visible for the first time. The plane had plowed a crater in a field near Roselawn, pointing to the northeast, scattering debris.
"It looked like everything had been put through a shredder," one witness said. "There was nothing there larger than a fist."
The only identifiable aircraft parts were the tail section and a portion of the fuselage.
Workers will begin the grisly task of removing human remains today, after allowing National Transportation Safety Board investigators time to begin their on-site work.
A temporary morgue that had been set up in Newton North High School was moved yesterday to an armory in Remington, Ind., so that school could resume today.
After a tour of the crash site, Secretary of Transportation Federico F. Pena challenged the characterization of the aircraft as a commuter craft and the implication that it does not meet the high standards of other airplanes.
David Hinson, head of the Federal Aviation Administration, who accompanied Mr. Pena, said the airplane, a 30-seat model called Super ATR-72, flies under the same regulations that govern major carriers.
Robert Crandall, chairman of AMR Corp., the parent company of American Eagle and American Airlines, said American Eagle aircraft must meet the same standards as the jets used by American Airlines and that the pilots are trained the same way American Airlines pilots are trained.
"We wouldn't put an aircraft in the air that isn't safe," he said.