In May, driver Joshua Brown became when his Tesla Model S crashed into an 18-wheeler with Autopilot engaged while the truck turned into an intersection. The factors that resulted in the crash have proven numerous and complicated. Now, Tesla says the reason the Model S didn’t “see” the truck was its automatic braking system, not Autopilot itself.

reported that during a Senate Commerce Committee meeting this week, an unnamed Tesla official cited a safety feature that is still relatively new but now vastly more common than semi-autonomous driving systems like Autopilot. But why they don’t co-exist, so to speak, is perplexing:

“Those systems are supposed to work together to prevent an accident,” said Karl Brauer, a senior analyst at Kelley Blue Book, an auto research firm. “But either the car didn’t know it had to stop, or it did know and wasn’t able to stop. That involves Autopilot and the automatic braking.”

The company told the committee staff that it considered the braking systems as “separate and distinct” from Autopilot, which manages the car’s steering, and can change lanes and adjust travel speed, the staff member said.

Using cameras, lasers, radar or some combination of those things, automatic braking systems detect when a forward collision with another car is imminent and apply the brakes for the driver. The safety feature now is available on a wide array of cars across all price ranges, and it .

In Brown’s Model S crash, his car was on Autopilot—reportedly while he , contrary to the car’s instructions to pay attention and keep hands on the wheel—and going when the vehicle struck a white 18-wheeler turning left in an intersection.

As the Times reports, neither Brown nor the car applied the brakes at the time of the crash, and the automaker has said “the radar and camera systems might have failed to detect the white truck against a bright sky.”

The wreck caused a great deal of controversy and discussion about semi-autonomous driving systems and how they are deployed on today’s cars. Along with new entrants from Volvo and Mercedes, the Model S is the closest thing to an autonomous car currently on the road, and is touted as the one with the most sophisticated semi-self-driving features.

Plenty of blame was heaped on Autopilot after the wreck. While I question the wisdom of Tesla’s subjecting customers to the “beta test” that is the feature itself, the idea that the wreck could have been caused by a much more common safety system could change the discussion. Then again, it’s curious why auto-braking apparently is “separate and distinct” from Autopilot. It raises a lot of questions, and questions we’ll be struggling to answer as semi-autonomous and autonomous cars become more common on roads.

I reached out to Tesla for a comment on this piece and will update if I hear back.