(Bloomberg) — Fiat Chrysler Automobiles issued a stern warning to employees after workers at two of its U.S. plants stopped production last week over concern management wasn't properly handling coronavirus risks. "Unauthorized work stoppages in our facilities create both disruption, and, potentially, safety concerns, and therefore cannot be tolerated," Mike Resha, Fiat Chrysler's head of North American manufacturing, wrote in a June 28 letter. Employees found to have instigated unapproved shutdowns will face disciplinary action, and stoppages "will result in zero pay," he wrote. FCA's threat following production pauses at key Jeep and Ram pickup factories shows tensions are still simmering at Detroit automakers, six weeks after they called workers back to the assembly line.