At a press conference at Grays police station, deputy chief constable Pippa Mills said police were unable to identify the bodies.
She said: "Please appreciate we are in the early stages of what is likely to be a lengthy investigation."
She added: "I'm unable to say at this stage how long the cordon will be in place.
LIVE UPDATES AS 39 DEAD BODIES ARE FOUND
"But please let me reassure you that my officers and partners will be doing everything they can to release the scene as soon as possible.
"This is an absolute tragedy and very sad day for Essex police and the community.
"We will continue to work alongside many other partner agencies to find out what led to these deaths.
"I would like to appeal to anyone who has any information to contact my officers at the major investigations team."
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