A body was found in landing gear

In an airplane of Delta Airlines, a dead body was found.
The airplane left NewYork and landed Narita airport.
A mechanic in the airport found a dead body in landing gear compartment,
while doing the maintenance.
The dead body was wearing a plaid shirt and jeans, and can't be identified who it is as yet.
Maybe he couldn't get out from the compartment while doing something?
If he was not dead while he was in the compartment, he must have died during the flight.
If so, that's terrible because the air gets very cold and there are not enough oxygen.
I wonder why he was in there.