Tesla Motors CEO Elon Musk tweeted Sunday that that he had a "promising call" with Bosch, a German auto-technology supplier that makes the radar sensor for Tesla's all-electric vehicles.Musk's tweet said that 'significant' upgrades might be possible for Autopilot, but did not specify what those might be. The improvements will eventually be added to vehicles with Autopilot capability via an over-the-air upgrade, Musk said. TwitterMusk tweeted that improvements are coming to Autopilot.Tesla is currently under investigation by the National Highway Traffic Safety Administration, and possibly the Securities and Exchange Commission, over several accidents that may have involved its Autopilot advanced driver assistance system (ADAS).The fatal accident, which took place on May 7 in Williston, Fla., resulted in the death of Joshua Brown, 40, of Canton, Ohio, when a tractor-trailer crossed both lanes of a divided highway in front of his Tesla sedan.Two other, non-fatal accidents connected to Autopilot are also under investigation. On Friday, a U.S. Senate Committee asked Tesla to brief it on the circumstances leading to the fatal accident involving Brown. Tesla has conceded that Autopilot was on at the time of the collision.For his part, Musk also thanked MobilEye for its help in "making Autopilot better." Netherlands-based MobilEye makes computer chips with image processing algorithms responsible for collision warning and prevention in ADAS, such as Tesla's Autopilot.While Tesla has remained fuzzy about how much of a role MobilEye plays in its in-house Autopilot ADAS technology, after the first Model S fatal crash in May, both Tesla and Mobileye stock went down.MobilEye later issued a statement that its technology is not currently able to recognize a crossing trailer but that it is working on an upgrade called "Lateral Turn Across Path (LTAP) detection" that will be available beginning in 2018.After thanking both MobilEye and Bosch for their roles in improving Autopilot, Musk asked that everyone "please direct all criticism at Tesla."