Call FBI. Take careful note of anything unusual.
Additionally, take note of any information that may prove useful later on in the event of follow-up after a terrorist attack. License plates, height, weights, distinctive features, etc. The circumstances in which the pictures were taken, combined with any other suspicious activity - or lack thereof - may help determine whether this is a false alarm or not. It is better to be safe than sorry at this point.
Hypothetically, if the building were demolished in a controlled explosion later on, what would your conscience have to say if you said noting? Better to calmly call the FBI, state that you do not want to over-react to anything but there may potentially be something of concern that you think could be suspicious but you are not sure about and would rather be safe and their professional opinion is valued in this case.
Give them as much accurate background detail in as scientific a manner as possible as possible as to why you were suspicious in the first place and let them make the decision as to whether this should be looked into further or not.
Thanks for the input. I'm going to get the apartment number from her tonight and report it.