If you encounter police in Massachusetts while wanted for a crime in another state, you will be arrested as a fugitive from justice.
As I’ve written in an earlier post, a fugitive from justice will be brought to the district court with jurisdiction over the area in which the arrest took place.
From there, the defendant can either contest rendition to the demanding state or, as often happens, he can simply waive his right to make such a challenge and thus expediate the rendition process.
But what happens if the defendant has charges pending in Massachusetts? The answer is found in M.G.L. c. 276, Section 20G:
If a criminal prosecution has been instituted against such person [i.e., a fugitive from justice] under the laws of this commonwealth and is still pending, the governor, in his discretion, may either surrender him on the demand of the executive authority of another state, or hold him until the final disposition of such prosecution or, if convicted and sentenced, until his discharge from imprisonment.
Therefore, it is up to the Massachusetts governor (or one of his executive agencies) to determine where the defendant will be prosecuted first.
In all likelihood, the defendant will not be rendered to the foreign state until he has resolved his charges here–that may include serving any sentence imposed on him for the Massachusetts crime.