There are many people who will use any scare tactic they can to steal money from others, including pretending to be an IRS agent.
Identity Theft Alerts
Due to the high volume of identity theft across the nation, federal and state governments are taking action to try to cut down on fraudulent returns.
The IRS will issue a 5071C which you can read about here: Understanding Your 5071C Letter
You can read about the notice from Alabama here: Taxpayer Identity Protection Program
Both have the option to verify your information online, over the phone, and Alabama sometimes requests that you mail a copy of your information to them.
Within the past few years, there have been a rash of scammers impersonating IRS agents using threatening phone calls and a claim along the lines of "give me a credit card number to pay your owed taxes now or go to prison". Modern fraudsters are well versed in "spoofing" phone numbers. That is, calling you from one phone or even a computer and it appears as another number on your caller ID. If you receive one of these phone calls, even if it supposedly comes from the IRS number itself, or if you want to prepare yourself in case you receive one, please read this article from the IRS website: Suspicious IRS Calls.
It wouldn't hurt to go over this article either: Tax Scams & Consumer Alerts
Also note that neither the IRS nor FBI will contact you by email about a secret inheritance or beneficiary payment from a deceased Nigerian man. If you are in the will of a Nigerian man, he will likely have proper lawyers use more conventional means of contacting you.
Be wary of a preparer who refuses to provide a copy of your tax return, or insists on having your refund deposited into their personal account on your behalf.