In addition to examining data, NHTSA took other measures, including surveying the crash site, and conducting road tests using a 2015 Tesla Model S 85D, which is similar to the auto that crashed, and a 2015 Mercedes C300 4Matic, as a "peer vehicle".
Joshua Brown, a 40-year-old former Navy SEAL from OH, was using the technology in his Model S before he collided with a truck near Williston, Florida, last May and was killed.
After a six month investigation into the death of a Tesla driver following a fatal crash in July 2016, USA auto safety regulators disclosed that they have found no evidence of defects in Tesla Motors.
The NHTSA investigations are only meant to determine whether a safety defect led to a crash - beyond that the agency does not attempt to assess the root causes of a crash.
The camera technology used by Tesla at the time was unable to recognize objects crossing in front of its cars, but the NHTSA said that Autopilot was primarily created to prevent rear-end crashes.
According to the agency, Tesla's crash rate dropped by nearly 40% after Autosteer - a component of the Autopilot system- was enabled.
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The agency said Brown "should have been able to take some action before the crash, like braking, steering or attempting to avoid the vehicle. The Autopilot system is an Advanced Driver Assistance System (ADAS) that requires the continual and full attention of the driver to monitor the traffic environment and be prepared to take action to avoid crashes".
Tesla in September unveiled improvements to Autopilot, adding new limits on hands-off driving and other features that its chief executive officer said likely would have prevented a fatality. Onboard computers crunch the data and allow the vehicle to aid the driver through actions such as adjusting course to stay in the center lane, or automatically braking to avoid a crash.
While the USDOT investigation has closed, another investigation of the same fatal accident by the National Transportation Safety Board is ongoing.
Tesla Model S and Model X vehicles both show dramatically reduced crash rates after Autosteer is installed.
Tesla won't be fined, but the agency criticized Tesla for calling the system Autopilot.
In the NHTSA report, it is explained that driving systems such as Autopilot require the complete and undivided attention of the driver.
Still, the report didn't completely exonerate Tesla from any future problems Autopilot might have. Tesla has called these statements "inaccurate" and that they stem from its "plans to develop its own vision system".