(Reuters) - The U.S. National Highway Traffic Safety Administration (NHTSA) said on Thursday it has closed its probe into 258 Zoox vehicles over a braking issue, after the Amazon ( AMZN ) self-driving car unit recalled them and updated their software.

The transport agency had opened the probe in May following two rear-end collisions that injured motorcyclists after the automated vehicles braked unexpectedly.

In March, Zoox agreed to recall 258 vehicles and said it addressed the braking issue by updating their software that made the vehicles brake unexpectedly when bicyclists or motorcyclists were near or approaching rapidly.

The probe was closed after NHTSA's Office of Defects Investigation analyzed Zoox's data demonstrating the resolution, the agency said.