A customer found what is believed to be a piece of human bone in a pair of socks at Primark.
Essex Police said the find at the Colchester store did “not appear be a result of recent trauma and had no skin or other particles surrounding it”.
The force said officers were liaising with the retailer, which was speaking to suppliers.
Primark apologised “for any distress caused” and said there was no evidence of an “incident” in the factory.
The discovery last month was reported to police on 2 January.
A police spokesman said: “Without further testing officers cannot be certain of the origin or age of the bone.
“Inquiries are ongoing.”
Primark said it was taking the matter “very seriously”.
It added: “No evidence of any kind exists to suggest that any incident has occurred in the factory, so it is highly probable that this object was placed in the socks by an individual for unknown reasons.
“Primark has been the subject of isolated incidents in the past which have subsequently been found to have been hoaxes.”