ATK has been around awhile. Back in the day they used to make some simple but yet competitive enduro and MX bikes using a Rotax motor. They were expensive but tough as nails if I remember right. The only "Made in the USA" off road bike. They bought Cannondale's remaining inventory after they sold out of the motorcycle market. On the flip side they are kinda rare and you don't see much about them so I couldn't tell you much other than what I already did. Is the one you're looking at the 605 DSES?
http://www.cifumotorsports.com/mc/1997atk/index.html
http://www.cifumotorsports.com/mc/1997atk/index.html