( 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 do n'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 . ''
NEW : Dead pilot used to engage in mock sword fights when in Turkish air force . Investigators : Turkish military plane dropped almost vertically to ground . Sudden drop because aircraft did not have enough forward speed , official says . Three Boeing employees killed , one remains hospitalized , company says .