If another company has registered the same trade name, you may still be able to register as well, as long as the other business or businesses provide dissimilar goods or services from yours. If there is not another company with the same trade name, it does not automatically guarantee you can use the name throughout the U.S. There may be pockets of the country where other unregistered businesses have common law trademark rights. This is where more searching is required.
This is simply a starting point for determining whether any future company growth will be stunted by a trademark infringement suit.