TORONTO - The chief executive of McEwen Mining is beefing up security at the Canadian company's mine refinery in Mexico to make it as "impenetrable" as Fort Knox after a band of masked gunmen made off with $8.5 million in gold.

"Everybody's a bit rattled by this," said Rob McEwen, recalling the early-morning heist at the company's El Gallo 1 mine on Tuesday.

"It's left people feeling vulnerable."

Three of the mine's security staff were driving to work in two separate cars when they were pulled over a short distance from the refinery by a crew of eight heavily armed robbers, McEwen said.

The gunmen bound and blindfolded the workers, taking their keys, identification, cellphones and cash.

"Somehow they knew who they were and had staked out an area and waited for them," said McEwen.

The robbers loaded the workers into a car and drove to the mine, where they entered through a back gate.

They apprehended several more people in order to get access to all of the keys needed to get through the security gates and armoured doors at the site.

"Everybody they captured was bound and blindfolded," said McEwen.

Victims who were wearing baseball caps had their hats pulled down over their faces and duct-taped in place.

McEwen said the robbers had a good sense of the refinery's layout, as well as exactly which individuals had the keys that would allow them to get through certain gates.

"They seemed to be quite well-prepared," McEwen said.

Gerard Vargas Landeros, the interior minister of Mexico's western Sinaloa state, where the facility is located, said the heist was most likely an inside job by either current or former employees.

The burglars loaded small parcels of gold concentrate — black powder that is melted in a furnace and made into partially refined gold bars — onto the back of a heavy-duty pickup truck and drove away, McEwen said.

In total, they made off with 900 kilograms of concentrate, containing roughly 7,000 ounces of gold.

McEwen said the company is "very thankful" that no one was hurt during the incident. But he noted that the heist traumatized some workers and cast a shadow over what's otherwise been a good year for the facility.

"We had a great first quarter and the second quarter was going great," he said. "Grades were up sharply, production was up, and then this happens."

The Toronto-based and TSX-listed miner still expects to produce 50,000 ounces of gold this year.

Local authorities are investigating, and McEwen said the government has been supportive.

"They're saying: 'We're going to get your gold back,'" he said.

—With a file from the Associated Press

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News from © The Canadian Press, 2015