The two U.S. fires started after the driver ran over debris in the road, damaging the battery pack that runs along the base of the Model S.
Tesla responded by raising the ground clearance at highway speeds and taking steps to prevent overheating of its charging systems, including giving customers upgraded wall adapters and providing charging-software upgrades.
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"Tesla's revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk," the NHTSA said on its website.
The regulator said that while a "defect trend" had not been identified, the closing of the investigation did not constitute a finding that a safety-related defect did not exist.
Also Friday, Tesla CEO Elon Musk announced in a blog post that the automaker has added a series of three shields, to protect the battery compartment, to vehicles manufactured after March 6. The automaker is also offering to retrofit existing vehicles upon request of the owner, or when the vehicle is brought in for regular maintenance, free of charge.
"During the course of 152 vehicle level tests, the shields prevented any damage that could cause a fire or penetrate the existing quarter inch of ballistic grade aluminum armor plate that already protects the battery pack," he wrote. "We have tried every worst case debris impact we can think of, including hardened steel structures set in the ideal position for a piking event, essentially equivalent to driving a car at highway speed into a steel spear braced on the tarmac."
"With a track record of zero deaths or serious, permanent injuries since our vehicles went into production six years ago, there is no safer car on the road than a Tesla. The addition of the underbody shields simply takes it a step further," Musk said.
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Tesla's shares were up more than 3 percent at above $214 in late morning trading on the Nasdaq.
—By Reuters; CNBC contributor Michael Strong also contributed to this report