There really is no way to know what they might be investigating. I STRONGLY suggest that you NOT POST any details about anything that you might even think would be the subject of an investigation. You should contact a competent, experienced criminal defense attorney immediately to assist you with this matter. And remember the right to remain silent is yours to invoke. You should consult an attorney immediately.
The information above, though authoratiative and based on years of education, training and experience is not intended as legal advice, nor does it create an attorney/client relationship between the responder and the poster of the question. As always, when you have a legal issue you should consult a licensed attorney in your jurisdiction.
You should consult with a criminal defense attorney prior to speaking to them. Additionally, you should not post anything on a public site that might be used against you.
I would call him back but not speak about anything or admit anything on the phone. If he asks you to set up a meeting, contact an attorney before you do anything. I can't speculate what it would be about.
You obviously know none of us can answer that question. I will assume the real underlying question is what you should do about the call. My strong recommendation is that you do not return the call yourself but instead have a qualified criminal defense attorney do that for you; three important benefits will result from that course of action. (1) you will not be able to say anything that could come back to hurt you, (2) the attorney can get the answer to the question you posted, and (3) your attorney will then be in a position to advise you on how to respond to the situation. You may very well not be at risk in the matter but you are much better off have legal counsel find that out for you. Good luck.
They could be calling you for anyone of several reasons. It could be they are investigating a potential crime in which you may have been involved or they belive that you may have been involved. They could be investigating a potential crime where you may be an unknowing victim or where they think you may have information about the crime but are not a target of the investigation.
The best thing to do is let them tell you why they are calling you and " we just want to ask you a few questions is NOT an acceptable answer" Then after they tell you what they want to talk about it would be best to retain counsel to communicate with them and do all of the talking for you. It really is best that you avail yourself of your right to remain silent and let counsel deal with this for you.
This information does not create an attorney /client relationship and should not be use or relied upon to make any decision in your case. Only consultation with your own attorney can provide you with the advice you need for your case.