After more than a dozen Tesla cars slammed into parked first-responder vehicles over the course of four years, US federal officials are stepping up their inquiry into the company's Autopilot feature.

The National Highway Traffic Safety Administration (NHTSA) announced on Thursday that its preliminary inquiry, which began in August, has been upgraded to an "engineering study," which is conducted before a recall decision is made.

The probe encompasses all four Tesla models — the Models Y, X, S, and 3 – accounting for around 830,000 vehicles sold in the United States.

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The probe is centered on Tesla's Autopilot feature, which is designed to assist drivers in navigating roads by using artificial intelligence to detect other vehicles. Although some drivers have used Autopilot while inebriated or in the backseat, the firm advises drivers to pay attention to the road and retain their hands on the steering wheel when using it.

The 16 crashes at the center of the investigation occurred between January 2018 and January 2022, with 15 people injured and one person killed. According to materials uploaded on the NHTSA website, forensic data suggests that the majority of the drivers had their hands on the steering wheel prior to impact and were following the system's instructions.

The probe is looking into whether the Autopilot feature ultimately impairs "the effectiveness of driver monitoring," according to the government.

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In a separate inquiry, the National Highway Traffic Safety Administration is looking into a number of reports about Tesla vehicles unexpectedly braking at high speeds, sometimes known as "phantom braking." Despite the fact that no wrecks or injuries have been reported, the government has received more than 750 complaints about the issue. The EPA has inquired about Tesla's knowledge of the problem.

For years, the government has been at odds with Tesla, particularly its outspoken founder, Elon Musk. The Washington Post reported earlier this year that anonymous officials and former regulators revealed the enraged response workers at the agency have received from Musk, and that regulators have had to learn to cope with a belligerent company.

Jennifer Homendy, chair of the National Transportation Safety Board (NTSB) – a separate federal agency that investigates crashes and makes recommendations to other agencies like the NHTSA – wrote to Musk last fall urging him to respond to the NTSB's safety recommendations for Autopilot that it issued to Tesla back in 2017. The NTSB suggested that Tesla implement protections to prevent drivers from operating vehicles in ways that are contrary to their design.

“If you are serious about putting safety front and center in Tesla vehicle design, I invite you to complete action on the safety recommendations we issued four years ago,” Homendy wrote in a letter to Musk.

It is worth noting that last year, Tesla recalled 675,000 cars in the United States and China, prompting new questions about how safe the vehicles are. 

The issues addressed in the recall are in the trunk and the front hood of two Tesla models - Model 3 and Model S.