"Tell the agent that you have an attorney and that 'my attorney will be in contact with you.'"
"Simply state that you will not discuss the matter at all without first consulting counsel and that counsel will be in touch with him."
"Just respond that you will consult with your attorney (or 'an' attorney) and that the attorney will be in touch."
"Simply repeat your mantra that you will not discuss the matter with him in the absence of counsel."
"You are not qualified to determine whether you have committed some crime or not"
I think this is the most important part. It could be that you did something but you thought it is not illegal at all.
The sentence regarding "my attorney will be in touch with you" should be the only thing you say.
In my very expensive experience with lawyers I have to say that they only care about my needs about 20% of the time, and usually these 20% happen before I sign their contract.
As much as it's probably correct to not fully trust an FBI agent it's also probably correct to not fully trust a lawyer. One needs to be able to defend oneself to a certain degree.