It could have been an inside job, but you have to consider that there's also a chance it may have been a well-planned heist.
Usually robbers that pre-meditate, plan, and execute a monumental heist such as this, aren't your run of the mill petty crooks. They're usually a little more sophisticated; (i.e. 'smarter', "most of the time.")
These individuals could have been laying low and hiding back in the shadows and methodically stalking the Armoured Car employee/employees.
They may have noted their daily route over a set period of days, weeks, etc. as to decide on when the best opportunity and moment presented itself.
Usually the odds of getting away with this type of high profile crime is not stacked in your favor.
The FBI agents are pretty damn good at their job when chasing down these types of bandits.
Only a handful of armoured car robberies have ever went unsolved.
With technology constantly advancing, they have a lot more weapons in their toolbox than ever before. That, and you also have to take into account that there are an increasingly amount of cameras that have been mounted all throughout cities and towns.
Not only do you have government/state/and or city municipality cameras, there are also private companies, businesses, and individuals who are located throughout an area where there's usually several that will record you at perfect vantage points.
Some of the footage that may have been captured provides the agents with crucial evidence to not only help in their man-hunt, but also to prosecute and convict these types of criminals.
If you've never seen a documentary about the Boston Marathon bombers, I would highly recommend taking time to watch a really good one.
Eyewitness accounts and camera footage that was captured and scrutinized throughout the city, led to the positive identification and subsequent pursuit and man-hunt that left one terrorist dead, and the capture of the second.
I'm probably preaching to the choir as yourself and millions of other people have probably already seen some sort of media of the chain of events that unfolded.
Through the hard work and dedication of not only law enforcement agents and officers who were right on top of this disaster from its fruition, but also to the sacrifices and and aid that many of our citizens who were involved put forth, we as a country should stand up and applaud them.
Many gave life and limb during this horrific attack on our country.
I appauld their interactions individually and also amongst each other that quickly put a stop to these terrorists who were hell-bent on killing as many Americans as they could.
It makes me proud knowing that we have men and women who dedicate their selves even if it meant their life, which several gave unselfishly.
May God be with them and their families.
Anyways, I didn't mean to leap-frog subjects, but hopefully you get my jest, which is; the FBI will more likely than not, capture these bandits who don't seem to have any regards to human life, which is evident in the fact that these robbers were menacing and brandishing AK-47s, which in turn makes you assume that they were going to get what they came for and nothing was going to stand in their way.