Library officials say a patron is suspected of selling hundreds of books, tapes and DVDs he had borrowed.
KCNC in Denver reports that 33-year-old Thomas Pilaar is suspected of using different names to get seven library cards from the Denver Public Library and checking out 300 items per card.
A library spokeswoman says it appears Pilaar was trying to sell all 21-hundred items.
She estimated the losses at about $35,000.
Library administrators in Arapahoe and Douglas counties think Pilaar struck there too.
Authorities were tipped by a woman who recently bought books through the online site Craigslist and noticed the library identification stamps.
The Denver district attorney's office says Pilaar is jailed on an unrelated parole violation and is being investigated for theft.
The Denver Public Library is reviewing its borrowing policies.