Best-selling author Greg Mortenson, whose claims about his charity have been challenged by CBS News, has canceled two speaking engagements for today at La Pietra, a private school.

Mortenson canceled Friday about noon, school spokeswoman Piia Aarma said.

“The only reason he gave was ‘unavoidable circumstances,’” Aarma said.

La Pietra paid Mortenson an undisclosed amount for the visit. Aarma said the speaking engagements will be rescheduled or the school refunded.

La Pietra declined further comment.

A “60 Minutes” investigation that aired Sunday alleged that “Three Cups of Tea,” a book co-authored by Mortenson that led to a humanitarian effort that is establishing schools for girls in Pakistan and Afghanistan, is filled with inaccuracies and that Mortenson’s charitable organization has taken credit for building schools that don’t exist.