(CNN) -- The Turkish Airlines plane that crashed this week in Amsterdam fell almost vertically to the ground, making only a short track in the muddy farmer's field where it went down, Dutch investigators said Friday.



Dutch investigators continue to probe the crash site for more clues.



That sudden drop indicates the aircraft did not have enough forward speed when it crashed, a spokesman for the Dutch Safety Board said, but the reasons for that are still unclear.



It is too early to speculate on the cause of the crash, spokesman Fred Sanders told CNN. Reports that it was caused by engine failure are premature, he said.



"There must have been ... reasons why the plane did not get enough speed," Sanders said. "We don't know yet why this came about, and that's the main thing that will have to be investigated."



Wednesday's crash of Flight 1951 from Istanbul, Turkey, to Amsterdam killed nine and injured more than 60 of the 135 people on board.



The crash, less than 500 yards short of the runway, split the plane into three parts.  Watch crash survivors return home »



Weather conditions at the time were favorable.



Passengers described feeling the plane suddenly drop before impact, and at least one passenger said he heard the pilot trying to give more power to the engines before the plane went down.



Safety Board specialists are analyzing the flight data and voice recorders and expect to finish their work at the crash site this weekend, Sanders said. Pieces relevant to the probe will be brought to a hangar at Amsterdam Schiphol Airport for further analysis.  Watch how survivors described crash »



The Safety Board may release preliminary findings next week, Sanders said.



The nine dead included five Turks and four Americans, said Theo Weterings, the mayor of Haarlemmermeer municipality, where the airport is located.  See where 737-800 has been involved in previous incidents »



There were four Boeing employees on the flight, and three were killed, the company said Friday in a posting on its Web site, citing the U.S. Embassy in Amsterdam as its source. The fourth employee is hospitalized.



The flight's pilot, Captain Hasan Tahsin Arisan, with more than 12,000 hours in the cockpit of Boeing 737s, also died.



Before becoming the pilot of passenger planes, Arisan was a Turkish Air Force squadron commander and F-4 Phantom Fighter pilot, famed for engaging in mock dog-fights with rival Greek pilots over the Aegean Sea.



An airline workers union mired in an ugly labor dispute with Turkish Airlines called Friday for the resignation of the company's top management, as well as the sacking of top government ministers.



Atilay Aycin, the chairman of Hava-Is which claims to represent 12,000 workers, denounced Turkish Airlines executives at a news conference Friday, accusing them of "failing to manage a crisis" and calling Wednesday's plane crash "work-related murder."



A week before the accident, Hava-Is published a statement that accused Turkish Airlines management of "inviting a disaster" by "ignoring the most basic function of flight safety, which is plane maintenance services."



After the crash, Turkish newspapers reported the Boeing 737 had had a series of malfunctions in the days and weeks before the fatal flight. Turkish Airlines defended its maintenance record.



"Turkish Airlines, which strictly follows its work on flight safety, followed all the maintenance procedures of the plane manufacturer [and] national and international authorities' directives for this plane," an airline statement said.



In its news release, Turkish Airlines noted a signal light and a wing-flap had recently been replaced on the aircraft.



Turkey's flagship airline is well-rated internationally for its overall safety record.



Some aviation industry experts in Turkey have played down the critical comments coming from Hava-Is, calling them overheated rhetoric.



"These are just political slogans," said Savas Sen, a Turkish Airlines pilot and the head of Turkey Airline Pilots' Association. "It's just like an argument between a child and his father."
What is the pilot's name? Where did the crash occur? How many people were killed? What measures are Turkish airlines taking to prevent this type of accident in the future? Where was the pilot previously employed? What caused the plane to drop suddenly? What happened to the plane?
Captain Hasan Tahsin Arisan, in Amsterdam nine strictly follows its work on flight safety, followed all the maintenance procedures of the plane manufacturer [and] national and international authorities' directives for Turkish Air Force did not have enough forward speed crashed
question: What is the pilot's name?, answer: Captain Hasan Tahsin Arisan, | question: Where did the crash occur?, answer: in Amsterdam | question: How many people were killed?, answer: nine | question: What measures are Turkish airlines taking to prevent this type of accident in the future?, answer: strictly follows its work on flight safety, followed all the maintenance procedures of the plane manufacturer [and] national and international authorities' directives for | question: Where was the pilot previously employed?, answer: Turkish Air Force | question: What caused the plane to drop suddenly?, answer: did not have enough forward speed | question: What happened to the plane?, answer: crashed