It isn't DHS. Well, DHS isn't running it and the helos aren't theirs. DHS might be involved logistically, but not operationally. This is nothing new, it's just only recently that stuff like this has been seen and noticed. The choppers are likely 160th SOAR Night Stalkers, and there are Tier 1/ODA personnel there as well. DEVGRU (Team 6) did it this past summer in Minneapolis and certain police personnel were made aware, but ordered to not disclose the information and to just tell people it was a "joint training exercise". The media reported that operations were going on, but they reported that it was Metro SWAT and DHS, when all local police did was cordon off areas to prevent people from having access to the areas where these ODAs were training. What they were doing was practicing high-rise takedowns, or DAs, for counter-terror and hostage rescue operations abroad. They may have also trained in the Metrodome and Target Center as well, besides working around buildings and parking structures. Most operations were done at night, but they did do some stuff during the daytime.
This stuff has been going on for years and they do it 2-4x per year in various cities. This is how they become proficient at urban warfare and counter-terror operations. I realize that the timing may be suspicious, but people need to keep in mind that this stuff is what allows them to save the lives of friendlies and kill bad guys in place where we don't want anyone to know that we've been.