Probe widens into mysterious plane crash in southern China

As investigators try to piece together clues on what caused a Chinese passenger plane to plunge from the sky Monday, officials told ABC News they are looking at every possible avenue, including an intentional downing.

None of the 123 passengers and nine crew members aboard Eastern Airlines Flight 5735 survived when it crashed into the mountains in the Guangxi region of southern China.

Early data shows the Boeing 737-800 plunged from 29,000 feet to 8,000 feet, leveled off and then went into a freefall, exploding into a fireball that was seen and filmed by people nearby.

On the ground, crews have found few clues as to what caused the disaster. The plane's black boxes haven't been recovered.

American intelligence agencies don't have a clear theory on what led to the plane crash, an official told ABC News. U.S. authorities studying satellite and radar data aren’t ruling anything out, including a possible intentional downing, the official told ABC News.

Chinese officials said Tuesday that the plane's crew didn't answer calls from air traffic control when they noticed the rapid descent during its flight from Kunming to Guangzhou.

Eastern Airlines grounded its fleet of Boeing 787-800 planes as a precaution.

The Civil Aviation Administration of China has called in the assistance of several international groups, including ones from the U.S., to find out what happened.

Members of the U.S. National Safety Transportation Board, the Federal Aviation Administration, Boeing and engine-maker CFM International are all joining the probe.

"The NTSB is in contact with the CAAC investigator-in-charge and we will support their investigation with our technical advisors from the Federal Aviation Administration, Boeing and CFM in all ways necessary," the NTSB said in a statement.

Alex Stone reports for ABC Audio:

ABC News' Amanda Maile contributed to this report.