DETROIT (AP) — Federal highway safety investigators want Tesla to tell them how and why it developed the fix in a recall of more than 2 million vehicles equipped with the company’s Autopilot partially automated driving system.

Investigators with the U.S. National Highway Traffic Safety Administration have concerns about whether the recall remedy worked because Tesla has reported 20 crashes since the remedy was sent out as an online software update in December.

The recall fix also was to address whether Autopilot should be allowed to operate on roads other than limited access highways. The fix for that was increased warnings to the driver on roads with intersections.

But in a letter to Tesla posted on the agency’s website Tuesday, investigators wrote that they could not find a difference between warnings to the driver to pay attention before the recall and after the new software was released. The agency said it will evaluate whether driver warnings are adequate, especially when a driver-monitoring camera is covered.