Sorry if you thought my reply rude. But I think it's the best advice I could give you. All anyone here can do is speculate on what the correct answer is.
Few here are qualified to give legal advice, and even fewer are willing to give it out for free in a public forum. Every webmaster (IMO) needs a lawyer, and these sorts of things should be submitted to them if they are of concern.
And in any case, every webmaster needs to take legal contracts seriously, and actually read them. I know it's standard practice for consumers to just click through an online legal agreement without reading it - I know I've done it myself. But when it comes to your website - it behooves you to actually read and understand them. And if you don't understand them, ask your lawyer (though that's not usually necessary). And maybe us, but realize that you're not getting good legal advice in that case.
No, didn't mean to be rude, just give the best advice I could give: read the contract yourself, don't ask us what we think it might say when we haven't even read it. An important webmasterly skill to acquire.
Now, once you know the answer, if it turns out that you have an option to attribute to a specific agency or not, you might want to ask the question again, in a slightly different way - SHOULD I?
It may not be as obvious as you think.
I can understand that you've paid good money for the stories, and you'd like to do with them as you please. So, if you are allowed to leave off the agency name, you'd like to.
But is that best?
You have to consider the prestige and authority added by the agency. It may add something that is missing is the articles are unattributed.
Will users believe that it's original material?
And if they do, will they take it with any authority?