Phase 1 of the inquiry, will look to determine if the takeover could result in “a substantial lessening of competition in any market”.
The CMA also said it has “reasonable grounds” for suspecting that it may be the case that JD Sports and Footasylum have “ceased to be distinct.”
The investigation has a 19 September deadline, where the CMA would reveal whether or not to escalate the takeover deal for a full phase 2 probe.
The sports fashion retailer announced the £90.1m takeover, after it had been given the go-ahead by shareholders on 25 April, after it was voted through at a meeting on 12 April.
Peter Cowgill, executive chairman of JD, said at the time of the offer: “We are pleased to make this Offer for Footasylum, which is very complementary to our existing businesses in the UK.”