The US league's probe cast a shadow over the midfielder's arrival, with MLS saying it would withhold details of a settlement between Inter Miami and LA Galaxy over Casemiro's discovery rights until the review concludes.
Inter Miami said they had signed the five-time Champions League winner - who left Manchester United at the end of last season - on a deal through the 2027 MLS Sprint campaign, with an option to extend to June 2029.
Yet the league then said in a statement that it had launched an investigation and was gathering all relevant information and would refrain from further comment until the review was complete.
MLS confirmed Inter Miami and the Galaxy had reached a settlement over Casemiro's Discovery Priority rights, but said the terms "will be released upon conclusion of the tampering investigation".
Under MLS rules, clubs must negotiate discovery rights with any team that previously listed a player before signing him outside the draft or free agency. The Galaxy had held rights to Casemiro before he opted to join Miami instead.
The league did not specify potential penalties or a timeline for when the investigation might conclude.
In their statement announcing the signing, Inter Miami said the club had acquired his discovery rights from the Galaxy as part of the deal.
Casemiro has made 91 appearances for Brazil and won the 2019 Copa America. He has played in three World Cups, starting all five of Brazil's matches in this year's tournament before they were eliminated in the round of 16 by Norway.
