TZARARACUA, Mexico -- A DC-9 jetliner fell from the sky in central Mexico, killing all 18 people on board shortly after most of its passengers had disembarked. Aviation investigators said Wednesday they had recovered the "black box" flight recorders.

"This is the most important thing of all," Andres Perez Zentella, Mexico's chief of aviation security, said of the flight recorders for Taesa Flight 725.Dozens of investigators, soldiers and police were searching the avocado field where jagged chunks of the plane had fallen, hoping to find clues on what brought the plane down.

The flight took off Tuesday from the western border city of Tijuana with 91 passengers aboard. After stopping in Guadalajara and Uruapan, it was carrying just 13 passengers and five crew members as it headed to Mexico City, 180 miles to the east.

One minute after the plane left Uruapan, the pilot radioed controllers to declare an emergency, according to Agustin Arellano Rodriguez, director of the state's Navigation Services in Mexican Airspace.

"We did not have any more contact," he said by telephone. "We never knew what the type of emergency was."

Accounts of the crash varied.

Witnesses said they saw a brilliant light in the sky and the debris fall to the mountains six miles southwest of Uruapan, said inspector Juan Alfonso Lara of the Michoacan state civil protection agency.

Agustin Gutierrez, Taesa's manager in Michoacan, said the plane nose-dived before crashing into the avocado plantation.

The plantation caretaker, Felipe Guzman, told the state news agency, Notimex, that the rear of the plane was on fire as it hit the ground and exploded. "After that, there were other explosions," he said.

Taesa spokesman Eduardo Cacho told a news conference the airline was not aware of any maintenance problems. He also said he had no information about communications between the crew and the flight control tower.

Because it was a domestic flight, the airline did not record the nationalities of the passengers.

In the Mexico City airport Tuesday night, crying relatives were led into a hangar where airline officials offered to take them to Uruapan.

Uruapan, a city of 250,000 dating back to the 16th century, is known for its avocado production. Tourists frequently stay there when visiting the Paricutin volcano, 20 miles to the west.

Taesa was founded in 1989 as an executive air charter service, and it expanded to commercial operations in 1991, using cut-rate fares to challenge established carriers Mexicana and Aeromexico.

Using the slogan "Taesa costs less -- much less," it sells tickets from booths in supermarkets and shopping malls.

The airline was badly hurt by Mexico's financial crash of 1994-95, and many of its debts were absorbed into a federal bank rescue program, where they have grown to about $450 million.

Competitors, suppliers and even the government have accused the airline of being slow to pay its bills. Chairman Alberto Angel Abed Shekaiban was arrested last year to be tried on charges of failing to turn over about $2.8 million in employee taxes.

Also today, the AFL-CIO filed a complaint against Taesa with the U.S. Department of Labor in Washington, accusing the airline of violating the North American Free Trade Agreement by allegedly firing flight attendants for union activities.

In June 1994, a Learjet operated by Taesa's charter service crashed near Dulles International Airport, killing all 12 aboard. The U.S. National Transportation Safety Board found that the probable cause of that crash was "poor decision-making, poor airmanship and relative inexperience of the captain."

In addition, the board noted that the plane had no flight data recorder, as required under international aviation rules. And while there were 10 passengers, the plane had only eight seats with safety belts installed.

In Seattle, Boeing Co. spokesman John Dern said Mexican authorities had requested assistance in the new investigation, and that the company would be involved, sending one person.

Dern said the plane was built in 1970 by what is now the Douglas Products Division of the Boeing Commercial Airplane Group.