MEXICO CITY, Mexico (CNN)  -- The Lear jet that crashed on landing last week in Mexico City, killing all aboard, including Mexico's interior minister, may have been felled by the turbulence from a large passenger jet it was following too closely, the nation's transportation secretary said Friday, citing results of a preliminary investigation.

The scene of the plane crash in Mexico City was one of panic and confusion, witnesses said.

Radar tapes show that the Lear 45 -- carrying three crew members and six passengers from San Luis Potosi -- was flying just 4.15 nautical miles behind a Boeing 767-300 at 6:45 p.m. November 4, Luis Tellez said.

The International Civil Aviation Organization calls for a separation of at least 6 nautical miles between a heavy jet like the Boeing and a medium-weight jet like the Lear to ensure that turbulence does not affect the smaller plane's control, Tellez said.

The flight controller in Mexico City recognized that the separation was insufficient and, at 6:44 p.m., told the Lear jet to reduce its speed, but "the Lear jet didn't begin to decelerate significantly until a minute and 12 seconds later," Tellez said.

"The transcript of the [cockpit voice recorder] reveals that, in this period, the plane entered a turbulence that surprised the crew members, and the pilot attributed it to the wake of the plane ahead," Tellez said.

The pilot asked the co-pilot, who had more experience, to take over, but he was unable to regain control, Tellez said.

"We know that the crew recognized the presence of the turbulence provoked by the wake of the plane that preceded it and, immediately after recognizing it, the plane gave a sharp turn and initiated its descent at a pronounced angle that culminated in its impact on the ground," he said.

The Boeing, which weighs 175 tons, is classified as a heavy plane. The 8-year-old Lear, which weighed 9.5 tons, was classified as a medium-weight plane, though it was on the light end of that scale, Tellez said.

"Therefore, it is more vulnerable than most medium-sized planes to the phenomenon of turbulence," he said.

Officials said last week that the left engine fell off the plane before it crashed.

Lab tests found no traces of alcohol or drugs in either pilot and no evidence of sabotage or explosives, he said.

Scrutiny of the wreckage has found no indication that anything was wrong with the plane or its engines, he said.

But the investigation did find "presumed deficiencies" in the pilot's certification to operate a Lear jet, Tellez said. Audio from the cockpit voice recorder "shows the lack of familiarity" of the pilot, Capt. Martin de Jesus Oliva Perez, with the instruments in the cabin, he added.

Among the plane's passengers was Interior Secretary Juan Camilo Mourino, the nation's second most-powerful official and a confidant of President Felipe Calderon's.

The plane crashed in Mexico City traffic and erupted in flames, killing five people on the ground and injuring 14 in addition to killing the nine people aboard.

Tellez said the final report, which will take several months to complete, will include studies carried out in a NASA simulator to confirm whether the wake was responsible.
Investigators found no evidence of what? What were the pilots unable to regain? What was too close to the 767? What was too close behind the Boeing 767? What did investigators find? Who did crash kill? Did they find drugs? What was the Lear jet too close to? Who couldn't regain control?
sabotage or explosives, control, Lear jet The Lear jet that crashed on landing last week in Mexico City, killing "presumed deficiencies" in the pilot's certification to operate a Lear jet, all aboard, tests found no traces of a Boeing 767-300 the co-pilot,
question: Investigators found no evidence of what?, answer: sabotage or explosives, | question: What were the pilots unable to regain?, answer: control, | question: What was too close to the 767?, answer: Lear jet | question: What was too close behind the Boeing 767?, answer: The Lear jet that crashed on landing last week in Mexico City, killing | question: What did investigators find?, answer: "presumed deficiencies" in the pilot's certification to operate a Lear jet, | question: Who did crash kill?, answer: all aboard, | question: Did they find drugs?, answer: tests found no traces of | question: What was the Lear jet too close to?, answer: a Boeing 767-300 | question: Who couldn't regain control?, answer: the co-pilot,