Well, she had a top secret job in Laurel, Maryland which means one thing: she worked for the National Security Agency (N.S.A.) whose headquarters is in Laurel. Definitely a targeted assassination, with two bungled previous attempts (the fake F.B.I. agent and the delivery man). Killing her inside her apartment would have been safer however apparantly her killers abandoned this idea since twice she wouldn't let them in so she was shot outside her apartment.
Why? We can only speculate but it would certainly be something related to her secret government work. She either found out something that she wasn't supposed to know (like finding out that 9/11 was an inside job) and wouldn't shut up about it, or since applied physics was her thing maybe she discovered something useful that her bosses wanted to patent without her so they get the royalties. Unsure of the exact motive but we can rest assured that both 1.it was something to do with her work and 2.her killers were from the N.S.A.