How is a public defender assigned to a case in Massachusetts

[Cemented]

This is related to the Tsarnaev case. How were Miriam Conrad, William Fick, and Timothy Watkins chosen for this particular case?

Were they randomly appointed by the office (the electronic version of pulling names out of a hat per say)?

Did Miriam Conrad (She is head of the US District Attorney's office) volunteer herself for the case? Do cases go to her, and she assigns defenders to any particular case?

Is it Miriam Bowler who assigned the public defenders?

If you know, can you cite a source please? Thanks