First, graduate from the State Police Academy and do some road time. After at least 3 - 5 years on the road (probably longer), apply when a slot opens up. There are two sides of this unit: the Fire and Explosion Investigation Section (arson investigators) and the Hazardous Devices Unit (a.k.a. the Bomb Squad). There is more info on the Mass. Department of Fire Services website.
To get a close up view of how the MSP Fire and Explosion Investigation Section operates, call (978) 567-3310 and ask about an unpaid internship.
Stm... Get in touch with Det. Paul Zipper. He works for the State Fire Marshall's Office, and usually has 1-2 interns at a time. I've been to three of his seminars and he's amazing and very informative, I learned a lot in his classes.
Not saying you should not go for this in MA but just and FYI. NH and RI have civilian Arson investigators that work for the State Fire Marshall's Office. I know from taking an arson class in NH a few (ok more then a few) years ago that the guys in NH are sworn in as NH Special State Police or something like that.
I would recommend staying in Mass. & hoping to do well enough on an exam to get on the job, then you can shoot for your goal of being a fire investigator.
I do not have anything against civilian investigators, but Massachusetts has an excellent reputation for being a front runner in Fire Investigations because we have local fire departments with good investigators & the fire marshals from the MSP). The team concept (Fire Department investigators/MSP Troopers assigned to the Marshal's Office) has proven its value.
Once we turn something over to the private sector we most likely will never get it back.
The pay in Mass. is also better than most states.............The above comments are just my opinions