A customer made the bizarre and grisly discovery in the Colchester store last December and reported the incident to police on 2 January.
The fragment is believed to be part of a phalanx, a bone in the finger.
At the time, Essex Police said it did ‘not appear to be a result of recent trauma’ and had ‘no skin or other particles surrounding it’.
Primark also apologised ‘for any distress caused’ and said that there did not appear to have been any evidence of an ‘incident’ at the factory.
Police have now said they have investigated as far as was ‘reasonably and proportionately possible’, with a police spokesperson saying: “At this time it is not linked to a criminal act.
“It is a possibility it was placed in the sock in its country of origin, but this cannot be confirmed.
“It did not appear to be a result of recent trauma and had no skin or other particles surrounding it.”
The force added that if any further information came forward, the case would be reviewed accordingly.
A spokesperson for Primark also said the company now considered the matter closed.
They said Primark had investigated the overseas factory where the socks were manufactured back in January, but found no evidence to suggest an incident had taken place there.
A statement said: “In January 2019 we carried out an investigation at the factory where these socks were made and found no evidence to suggest any incident had occurred there.
“It is highly probable that the 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.
“Following our own and the police investigation, we consider the matter closed.”
Recently, supermarket giant Tesco was forced to suspend production at a factory in China after a six-year-old girl found a message claiming to be from prisoners in Shanghai in a box of Christmas cards.
Inside one of the cards, a message in capital letters read: “We are foreign prisoners in Shanghai Qingpu prison China.
“Forced to work against our will. Please help us and notify human rights organisation.”
The message then asked the reader to contact former journalist Peter Humphrey, who has previously spent time in Qingpu prison.
Tesco said it was ‘shocked’ and has since halted production at the factory that produces the particular cards, also launching an investigation.
The company also said it would de-list ZheijiangYunguang Printing – the supplier of the cards – if it was found to be using prison labour.
A Tesco spokeswoman said: “We were shocked by these allegations and immediately halted production at the factory where these cards are produced and launched an investigation.”