Legal Question in Intellectual Property in New York

It looks like you've come across something that's known as an "orphan work." It's a real problem. Various efforts in Congress and in litigation (particularly relating to the Google Books digitization project) have been made during the past several years but have failed to produce a comprehensive solution.

When you refer to "using some material" it fails to give enough information as to the nature of the use to enable me to provide much further comment. Best thing for you right now would be to search the Internet for "orphan works" and the "Google Books" litigation and learn your options from that.

Mr. Elman's response is very true and you can't guarantee no one will appear later as the rightful owner; however, there is one option you may pursue as to this one company. You can contact their legal representation, talk through the situation, and ask them to sign a release of liability. Such a release would be a contract between you that they won't sue you. Contracts do require "consideration" meaning that you will have to give them something in return for their promise not to bring action against you. If such a meeting of the minds is not possible, a simple signed letter stating that they are not the owner of the work should suffice. The letter must come from their legal representative or an officer who has the authority to sign such documents - a secretary or office worker is not good enough.

Keep records of all of your efforts to find and clear this potential infringement. Such due diligence may, at the very least, mitigate any damages from subsequent litigation. This could reduce the statutory damages to $200 per work sold. Copyright infringement damages can be steep and likely more than you will make by selling your work. The best option is to avoid using an orphaned work.