WASHINGTON, May 31 (Reuters) - The U.S. National Highway

Traffic Safety Administration said on Friday it is seeking more

information in its investigation into Amazon.com's ( AMZN )

self-driving Zoox vehicles due to unexpected braking leading to

two rear-end collisions.

The agency, which opened a probe into the crashes this

month, said it is seeking video of the crashes and documents

from Zoox related to the vehicles.

NHTSA said it "is concerned that vehicles exhibiting

unexpected rapid braking may increase the risk of crash" and

added the "risk is particularly acute to road users behind the

Zoox vehicles who are unable to reasonably anticipate or react

to the unexpected sudden braking."