This is my first post but Your not the first to be accused of alleged infringement as I and others have been accused but please do not panic
I can point you to a site that has collated a lot info about this matter http://beingthreatened.yolasite.com/
And are still are hard at work getting info and fighting this whole mess ... please do not ignore the letters as they are legal and most definitely not a scam.
Richie