In Alabama, if there is a federal lien on property sold for unpaid real estate taxes, then the IRS (or whatever government agency has the lien, such as Medicaid, SBA, etc.) has the right to redeem from the tax sale in order to protect their own lien. If they attempt to foreclose on their lien without first redeeming, they will not have good title. They MUST redeem. They cannot simply take your tax sale property away from you.
The investor is obligated to provide certified mail notice to all lien holders. This includes mortgage lenders, IRS, judgment creditors, everybody. There is no time limit for WHEN to give the notice. But, whenever the investor gives the notice, the lien holder has one year from that date to redeem, or the regular 3 years that everybody has after the tax sale, whichever is longer. If you give your notice one month after the tax sale, the lien holder has 2 years and 11 months left to redeem. If you give it 3 years or 5 year or 15 years after the tax sale, the lien holder has one year from that date to redeem.
Where most investors get in trouble is, they never give the required notice to lien holders. As a result, long after the investor has a tax deed and thinks they are safe, a lien holder can pop up and redeem the property. This is especially dangerous if the lien holder has made any improvements to the property. If the property contains a residential structure, then a redeeming party will also have to pay the value of what is called preservation improvements. If the property is vacant land, or if it is commercial property, the redeeming party has to pay only for taxes and interest. Even if the property contains a residential structure--a house for example--and the investors adds on to it or upgrades the finishes, the redeeming party does not have to pay for those. Just preservation improvements, which basically means repairs. As a result, an investor could lose a lot of money if it does not send out those notices, and then the IRS or some other lien holder redeems.
If you want to speed things up and not wait a year, the IRS will usually release its redemption rights on a tax sale property, without being paid anything at all. Here is a link for an IRS publication that explains the process: http://www.irs.gov/pub/irs-pdf/p487.pdf