Musk expressed relief that nobody was near the point of ignition when it happened, and no equipment was significantly damaged during the fire. He also suggested that employees look out for negligence or sabotage, writing "Please be on the alert for anything that's not in the best interests of our company" and reminding employees of former Intel CEO Andy Grove's famous maxim: "only the paranoid survive."

Tesla CEO and chairman Elon Musk sent an email to employees on Monday morning explaining that the factory had to stop their "body production line" for several hours on Sunday night due to a "small fire." On this part of the line, a car body's sheet metal has been welded together but the body has not yet been painted.

Tesla continues to struggle with fires at its electric vehicle plant in Fremont, California, according to correspondence reviewed by CNBC on Monday.

A Tesla spokesperson confirmed the email and said, "Last night, there was smoldering in an air filter in the welding area of the body line. The smoldering was extinguished in a matter of seconds. There were no injuries or significant equipment damage, and production is back online."

CNBC previously reported that the paint shop in Tesla's Fremont factory had experienced at least four fires since 2014, including one in April that shut down production.

Tesla stock has rallied more than 20 percent since the company's annual shareholder meeting on June 5. There, Musk promised investors that Tesla would become profitable and cash flow positive in the second half of 2018, and that Tesla would soon be able to produce 5,000 Model 3 electric cars per week.

Last week, Tesla announced a broad restructuring, slashing at least 9 percent of its workforce. Workers who are actively involved in Model 3 production would not be affected, the company said.

Here is the full e-mail about the most recent fire: