Worrisome Signs From Dept of Homeland Security

I don't trust Alex Jones or his site Infowars.com, due to their 911 trutherism and general hysteria over imaginary plots and pogroms, but this report got my attention and seems to be well supported and referenced:  DHS Purchases 21.6 Million More Rounds of Ammunition.

Why is the Department of Homeland Security buying tens of millions of bullets?  Why are they buying the so-called "assault weapons" that are to be denied to ordinary citizens?

The article also states:

Such massive quantities of ammo purchases have stoked fears that the agency is preparing for some kind of domestic unrest. In 2011, Department of Homeland Security chief Janet Napolitano directed Immigration and Customs Enforcement to prepare for a mass influx of immigrants into the United States, calling for the plan to deal with the “shelter” and “processing” of large numbers of people.

What immigrants does Big Sis have in mind?  Why hasn't there been a congressional investigation into these actions by the DHS?