Prime Inc trying to collect for training from 5 years ago

Hello everyone I was with Prime, Inc. for about three months from September 2013 to November 2013. I went through their on-the-job training and received my CDL and continued on with a trainer before realizing being away from home wasn't for me and I decided to leave the company. I went back to the main terminal and let them know I was leaving, at that time they let me go but never said anything about paying them for the CDL. Jump forward to today I just received a one page report stating I owe $3,725.19. It doesn't say what this money is owed for, all it says is it is due in one month and (This is an attempt to collect a debt. Please refer to the Payroll Settlements for itemized detail of the debt. If you dispute the validity of the debt, you have 30 days to notify of such).

My question for you all is; it has been 5 years since I left Prime, Inc. and this is the first contact I have received from them. There was no other included information or reports to refer to in order to see this "itemized detail of debt". This "letter" I received is very vague and in no way does it look like a legal document or invoice. There is no collection on my credit reports nor has there ever been.

What do you guys in the field think of this? I have attached the picture of the letter I was sent

 

Thank you for all the responses I wanted to get some general information before calling the number the letter gave me. I am still in the business but with a local company. I will call and ask for the contract with my signatures stating that if I leave within X amount of time I owe X amount to them. If they have all this then yes I will of course pay the amount training isn't free and I agree and understand that it was just very odd to me that this is the first I have heard from Prime Inc after 5 years away, and the fact that the letter doesn't directly state what this is in regards to.