Why couldn't the FBI land this case by itself? What are the responsibilities and special investigative powers (if any) of "Homeland Security Investigations." How do they justify their existence independent of the FBI? Their webpage suggests they do almost exactly the same thing.
ICE is the agency that handles customs inspections, and presumably get called in for any type of smuggling of contraband that crosses national borders.
It would make a lot of sense for the Federal government to have a single agency that investigates all computer-related crimes, but since that would make sense it will never happen. Instead each agency (FBI, US Secret Service, ICE, etc.) will investigate computer-related crimes that tie into their bailiwick, likely cooperating for cases that cross jurisdictional boundaries like this case did.
http://www.ice.gov/hsi