So they misled investigators throughout multiple cities across the United States. Therefore, it's a federal issue. This really isn't that hard to understand.
No. Why it can be made a federal issue has been successfully outlined by others in this thread. Misleading investigators throughout multiple cities, however, isn't the reason.
It seems to be when it's coordinated by single organization across state lines. That's what turns it from a series of totally unrelated crimes in different states into one nation-wide criminal operation.