Bill Barr can request Military assistance in all capacities, including investigations, which would set up the military commissions.
https://www.law.cornell.edu/uscode/text/28/535
28 U.S. Code § 535.Investigation of crimes involving Government officers and employees; limitations
(a)The Attorney General and the Federal Bureau of Investigation may investigate any violation of Federal criminal law involving Government officers and employees—
(1)notwithstanding any other provision of law; and
(2)without limiting the authority to investigate any matter which is conferred on them or on a department or agency of the Government.
(b)Any information, allegation, matter, or complaint witnessed, discovered, or received in a department or agency of the executive branch of the Government relating to violations of Federal criminal law involving Government officers and employees shall be expeditiously reported to the Attorney General by the head of the department or agency, or the witness, discoverer, or recipient, as appropriate, unless—
(1)the responsibility to perform an investigation with respect thereto is specifically assigned otherwise by another provision of law; or
(2)as to any department or agency of the Government, the Attorney General directs otherwise with respect to a specified class of information, allegation, or complaint.
(c)This section does not limit—
(1)the authority of the military departments to investigate persons or offenses over which the armed forces have jurisdiction under the Uniform Code of Military Justice (chapter 47 of title 10); or
(2)the primary authority of the Postmaster General to investigate postal offenses.
(Added Pub. L. 89–554, § 4(c), Sept. 6, 1966, 80 Stat. 616; amended Pub. L. 107–273, div. A, title II, § 206, Nov. 2, 2002, 116 Stat. 1779.)
"the authority of the military departments to investigate persons or offenses over which the armed forces have jurisdiction under the Uniform Code of Military Justice"
https://www.law.cornell.edu/uscode/text/10/280
10 U.S. Code § 280 - Enhancement of cooperation with civilian law enforcement officials
(a)The Secretary of Defense, in cooperation with the Attorney General, shall conduct an annual briefing of law enforcement personnel of each State (including law enforcement personnel of the political subdivisions of each State) regarding information, training, technical support, and equipment and facilities available to civilian law enforcement personnel from the Department of Defense.
(b)Each briefing conducted under subsection (a) shall include the following:
(1)An explanation of the procedures for civilian law enforcement officials—
(A)to obtain information, equipment, training, expert advice, and other personnel support under this chapter; and
(B)to obtain surplus military equipment.
(2)A description of the types of information, equipment and facilities, and training and advice available to civilian law enforcement officials from the Department of Defense.
(3)A current, comprehensive list of military equipment which is suitable for law enforcement officials from the Department of Defense or available as surplus property from the Administrator of General Services.
(c)The Attorney General and the Administrator of General Services shall—
(1)establish or designate an appropriate office or offices to maintain the list described in subsection (b)(3) and to furnish information to civilian law enforcement officials on the availability of surplus military equipment; and
(2)make available to civilian law enforcement personnel nationwide, tollfree telephone communication with such office or offices.
(Added Pub. L. 100–180, div. A, title XII, § 1243(a), Dec. 4, 1987, 101 Stat. 1163, § 380; amended Pub. L. 100–456, div. A, title XI, § 1104(a), Sept. 29, 1988, 102 Stat. 2046; renumbered § 280, Pub. L. 114–328, div. A, title XII, § 1241(a)(2), Dec. 23, 2016, 130 Stat. 2497.)
"The Secretary of Defense, in cooperation with the Attorney General, shall conduct an annual briefing of law enforcement personnel of each State (including law enforcement personnel of the political subdivisions of each State) regarding information, training, technical support, and equipment and facilities available to civilian law enforcement personnel from the Department of Defense."