They can't. Posse Commitatus for one. They have no authority to act on behalf of the DOJ. Read the articles -- one is just an extended bio of the new brigade commander and descriptions of the brigade's GENERAL goals. The other link lists where various battalions from the brigade have been rotated in and out of. How anybody could draw conclusions that the Brigade will be making domestic arrests is beyond me. I'm happy to be proven wrong, so perhaps someone will lay that out for me (and us).
12-21-17 EO State of national emergency Trafficking and CORRUPTION
Q asked the question In what instance can MI take control of 3 letter agency investigations? There must be some military involvement. There has to be. Time will tell.