Tesla Motors has told Senate investigators that its crash-prevention system failed to work properly in a fatal crash, but said its Autopilot technology was not at fault, according to a Senate staff member.

Instead, Tesla told members of the Senate Commerce Committee staff on Thursday that the problem involved the car’s automatic braking system, said the staff member, who spoke on condition of anonymity.

It was not clear how or why Tesla considers the automatic braking system to be separate from Autopilot, which combines automated steering, adaptive cruise control and other features meant to avoid accidents. Tesla declined to comment on Friday.

“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.”