I applied online and was contacted by a recruiter who set up three back-to-back phone interviews. I basically answered every single question, needing only a slight push a couple of times, which was always sufficient. Then, a couple of days later, I received a devastating rejection letter. Is it possible that it's just too late in the recruiting process, so they don't have many interview positions left? Or did I simply not do as well in the interviews as I thought?

Signed a non-disclosure agreement, but the topics were algorithms and data structures. You can be extremely well prepared by looking at the questions posted on Glassdoor.