"Securing them is absolutely critical to safety," said Steven Wallace, former director of the FAA's Accident Investigation Unit. It's critical that the total weight be within the limit and that the plane be balanced, he said.
"When the airplane is rotated with the nose up, the cargo moves aft if it's not properly secured," he said.
National Air Cargo said the cargo was properly loaded and secured and had passed all necessary inspections prior to departing Camp Bastian. No additional cargo was added during the stop in Bagram and the aircraft's cargo was again inspected prior to departure.
The company would not speculate on a possible cause of the crash and said it was cooperating with investigators.
Six of the crash victims were from Michigan, National said. They were identified as Brad Hasler, Jeremy Lipka, Jamie Brokaw, Rinku Summan, Michael Sheets and Gary Stockdale.
The other victim, Timothy Garrett, was from Kentucky.
Taliban spokesman Zabiullah Mujahid said his group shot down the aircraft and that "several foreign soldiers were killed."
That claim could not be verified. A number of Taliban claims of responsibility have proven false.