Could be "anything," but the first thing Bloomberg thinks of are those who don't like ObamaCare, presumably conservatives or Tea Party activists.
Audio: MP3 clip 
From part of Couric's interview with the mayor aired on the Monday, May 3 CBS Evening News:
KATIE COURIC: Law enforcement officials don't know who left the Nissan Pathfinder behind, but, at this point, the mayor believes the suspect acted alone.CBSNews.com version  of Couric's interview.
MAYOR MICHAEL BLOOMBERG: If I had to guess - 25 cents - this would be exactly that, somebody-
COURIC TO BLOOMBERG: A home-grown?
BLOOMBERG: Home-grown, maybe a mentally deranged person or somebody with a political agenda that doesn't like the health care bill or something. It could be anything.
- Brent Baker is Vice President for Research and Publications at the Media Research Center. Click here  to follow him on Twitter.