In an article published yesterday on the website Alternet, the researchers find that a whole set of contributions made during the 2007-08 election cycle are no longer listed in the Federal Election Commission (FEC) files.
Update 7/18/12: As a result of the article, the FEC acknowledged their mistake, and restored the missing data to the public record. Thank you, Paul!
You can read the FEC press release on the matter here.
Paul and his collaborator respond to the press release here.