The National Highway Traffic Safety Administration has closed its investigation into recent Tesla car fires, as the Palo Alto electric vehicle company announced another voluntary effort to improve safety. The NHTSA launched the probe following a pair of fires late last year that occurred after Model S sedans ran over roadway debris, but it said in its report that a “defect trend has not been identified.” It cautioned, however, that ”the closing of the investigation does not constitute a finding by NHTSA that a safety-related defect does not exist, and the agency reserves the right to take further action if warranted by new circumstances.” In a separate but clearly related announcement, Tesla CEO Elon Musk said Friday the company will outfit all new cars and retrofit existing ones for free with a “triple underbody shield” to reduce fire risks.