MIT Lincoln Laboratory interview questions

The first interview was with a recruiter on campus during the college career fair. After talking to me for about 10 minutes and quickly looking through my resume, an interview was scheduled for the next day. During that interview I was mostly asked to explain my research and how it might be relevant to work done at LL. I was then advised to fill out an online application, being sure to note my school code, and apply for all positions that show up when searching for my major. I was contacted by a different employee a month later, during which I inquired about the status of my application multiple times with the HR and the recruiter. This was a phone interview, where again I was asked about my research, as well as various specifics of my background--including course work, programming experience, etc. At the end of the phone interview, I was told that I would be contacted about scheduling an onsite interview, and sure enough, about a week after the phone interview someone from the HR contacted me about scheduling my visit. The scheduling was very smooth, with LL putting nearly all travel expenses (car rental, hotel, airfare) on their company card, and I got reimbursed for the remaining expenses very promptly. The onsite interview lasted a day and a half: I talked to people from 3 different groups, including the group leaders as well as staff at my level, and I also gave a presentation about my research on the first day. As a side note, I was asked to send my abstract at the last minute (same day as my flight), and also email my slides, again at the last minute (the night before), so make sure you check your email frequently before the interview. Most of the people I talked to were very nice, and didn't grill me--mostly I was asked about my research projects, experience, and interests. There were a couple of cliched questions such as "where do you see yourself in five years" as well. The group I interviewed with on the second day was more intense, as apparently none of the group members were able to attend my talk, and as a result I had to essentially give my talk in five minutes over and over again (I admit it got tiring and annoying at some point). One of the group members asked me a detailed technical question--the only one during my interview process--about the Fourier transform of the signal coming from a radar detector under some conditions (I don't remember exactly). I can't do Fourier transforms in my head and I didn't answer that very well. Some of the people I interviewed with asked about my national origin, even though I believe that it's illegal of them to do so. I am not sure why they were asking that, I suppose out of curiosity. My application was rejected a month later, even though I kept contacting the HR contact person periodically to check up on the status of things, and even informing them of another job offer I received did not speed things up. Overall, LL made a very positive impression on me, but I have no idea why their hiring process is so slow, and why it takes them so long to reject an applicant.