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.

In a l etter 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 sent out. The agency said it will evaluate whether driver warnings are adequate, especially when a driver-monitoring camera is covered.

The agency asked for volumes of information about how Tesla developed the fix, and zeroed in on how the company used human behavior to test the recall effectiveness.

The 18-page letter asks how Tesla used human behavior science in designing Autopilot, and the company's assessment of the importance of evaluating human factors.

It also wants Tesla to identify every job involved in human behavior evaluation and the qualifications of the workers. And it asks Tesla to say whether the positions still exist.

Tesla is in the process of laying off about 10% of its workforce, about 14,000 people, in an effort to cut costs to deal with falling global sales. CEO Elon Musk is telling Wall Street that the company is more of an artificial intelligence and robotics firm rather than an automaker.

NHTSA said it will evaluate the “prominence and scope” of Autopilot’s controls to address misuse, confusion and use in areas that the system is not designed to handle.

It also said that Tesla has stated that owners can decide whether they want to opt in to parts of the recall remedy, and that it allows drivers to reverse parts of it.

Safety advocates have long expressed concern that Autopilot, which can keep a vehicle in its lane and a distance from objects in front of it, was not designed to operate on roads other than limited access highways.

Tesla tells owners that the system cannot drive itself despite its name, and that drivers must be ready to intervene at all times.