Staff at the nature reserve close to where a military helicopter crashed gave emergency services advice when they first arrived on the scene.
The head of the wildlife trust which runs Cley Marshes reserve said he initially feared two people on the ground could have been killed.
Brendan Joyce, chief executive of the Norfolk Wildlife Trust, said: 'I was concerned when I heard four people had lost their lives. The initial reports were it was an Apache helicopter, and they only fly two, so I wondered who else was involved.'
He added: 'My understanding is that it came down on the shingle bank, so not on the actual reserve. We don't know what the cause was at all.
'It would have been dark. There would not have been any staff or volunteers on the site at the time.
'I do know that our staff locally initially assisted in terms of advice to the emergency services.
'Obviously the emergency services and military have taken over pretty quickly and have sealed the area off. We are not involved.
'We are deeply shocked by it and our first thoughts are with the families of those who have lost their lives.'
He said he expected the area to be cordoned off for some time while the military investigate what happened.
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