The move is unusual because normally the Justice Department looks into takeovers which harm competition.
The deal announced by Microsoft in January is its biggest-ever and is set to be the largest all-cash acquisition on record. It will bolster Microsoft's firepower in the booming videogaming market where it takes on leaders Tencent and Sony.
Microsoft has so far avoided the type of scrutiny faced by Alphabet and Meta Platforms, but this deal, which would make it the world's third-largest gaming company, will put the Xbox maker on lawmakers' radars.
Reuters previously reported that Microsoft would pay a $3 billion break-fee if the deal falls through, according to a source familiar with the matter, suggesting the company was confident of winning antitrust approval.
Microsoft and Activision gave themselves until June 2023 to complete the transaction.