DUBAI, United Arab Emirates (AP) -- A U.S. university professor who advised a Chinese-American researcher sentenced to 10 years in prison in Iran is defending his former student as innocent of all charges against him.

Princeton University professor Stephen Kotkin told The Associated Press by email on Monday that Xiyue Wang is a "remarkable, linguistically gifted graduate student" who studied governance in 19th and early 20th century Muslim regions.

Iran's judiciary announced on Sunday that Wang had been accused of "infiltrating" Iran and passing confidential information to the U.S. government and research institutions abroad. He was sentenced to 10 years in prison.

Kotkin says the documents Wang collected in Tehran were 100 years old.

Copyright 2017 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.