The incident happened at New Yard Landscaping in Hopkinton, N.H., early Tuesday, the Boston Herald reported.
When police arrived on the scene, the two men, identified as Lucas Bourke and Ethan Keeler, both 21, were dead.
Officials said they believe the victims were using an oxy-acetylene torch to open the strongbox.
It was unclear whether they knew what was in the box before they tried opening it, officials said.
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