The University of Sydney confirmed to NBC News that Endicott was enrolled there and said it was in touch with his family. It declined to comment further.
The American Consulate in Sydney, meanwhile, said it was "aware of reports" of a missing American citizen and would "fully cooperate with local authorities."
Ackerman was a star athlete who graduated in the Class of 2014 at Robinson High School in Virginia.
Cassandra Vinograd is a Senior Writer and News Editor. Before joining NBC News, she worked as a London-based correspondent for The Associated Press and specialized in politics, foreign affairs and defense.
Vinograd previously worked as an editor for The Wall Street Journal in Brussels and London.
She has reported extensively from Afghanistan and on West Africa and the Middle East.