Attorney General Merrick Garland appointed Jack Smith as a “Special Counsel” with “the full power and independent authority to exercise all investigative and prosecutorial functions of any United States Attorney” to investigate Trump.
The potential problem is that the Constitution requires Senate Confirmation of Officers of the Unites States or that the position is an Inferior Officer created by law.
While DOJ has Special Council Regulations (28 CFR Part 600 - PART 600—GENERAL POWERS OF SPECIAL COUNSEL) there is currently no law authorizing Special Counsels.
Andy McCarthy, former US Attorney, has a good review article describing the issue.
https://www.nationalreview.com/2024/06/merrick-garlands-special-counsel-appointment-of-jack-smith-is-in-peril/
Two former US Attorney General’s have filed a brief arguing that the appointment of Jack Smith is not Constitutional.
Stateshttps://storage.courtlistener.com/recap/gov.uscourts.flsd.648653/gov.uscourts.flsd.648653.364.1.pdf
Appointments-clause scholar Seth Barrett Tillman have filed a brief arguing that the appointment of Jack Smith is not Constitutional.
https://reason.com/wp-content/uploads/2024/03/2024-03-21-Tillman-Brief.pdf
https://x.com/optimistcskeptc/status/1805982696239149515?s=46&t=Y4fVsDnz2q8uZ3VNKeco4A