Chilean authorities are investigating the violent theft of several copper-filled shipping containers from the national firm Codelco.

Local prosecutor Juan Carlos Catalan said in a statement to Reuters that 10 armed men entered the San Antonio port early on Tuesday morning, attacked workers and stole 13 containers, 12 of which had copper.

“There was one guard and four workers that [the assailants] tied up and beat and left locked up,” Catalan said.

Workers alerted authorities after they freed themselves.

Investigations are still ongoing.

Suspects have not yet been detained.

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Codelco said the copper was scheduled to be exported and was insured.

Local media reported that the copper plates were worth an estimated $4.4 million.

Mining companies in Chile have repeatedly complained about copper thefts by organized gangs.

Codelco is a Chilean state-owned copper mining company.