WASHINGTON, May 2 (Reuters) - The U.S. National Highway Traffic Safety said Wednesday that a prior investigation into Tesla Inc’s semi-autonomous “Autopilot” self-driving system did not assess the “effectiveness” of the technology.

In 2017, NHTSA closed a probe into a May 2016 fatal crash involving a driver using the system and cited data from the automaker that crash rates fell by 40 percent after installation of Autopilot’s Autosteer function. Tesla has repeatedly cited the statistic in defending the system. NHTSA said Wednesday that its crash rate comparison “did not evaluate whether Autosteer was engaged.” The agency added that it “performed this cursory comparison of the rates before and after installation of the feature to determine whether models equipped with Autosteer were associated with higher crash rates, which could have indicated that further investigation was necessary.” Tesla did not immediately comment Wednesday. (Reporting by David Shepardson; Editing by Lisa Shumaker)