People pass by the Tesla New York City store in the Chelsea neighborhood of New York March 19, 2014.??[Photo/Agencies]

US safety regulators said they closed an investigation into fires involving electric sports car maker Tesla Motors Inc's popular Model S sedans after finding no "defect trend".

The US National Highway Traffic Safety Administration (NHTSA) began its investigation after two cars in the United States and one in Mexico caught fire.

The two US 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.

"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.

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