The agency has paused an "informal" 180-day transaction clock "to allow for thorough staff and third-party review" of recently submitted materials.
Sprint and T-Mobile have gone down a rocky road to a merger, calling off and resuming talks. The companies announced that they would merge last April in a bid to cut costs and combine forces to develop a next-generation network called 5G, which would provide faster speeds, more capacity and lower response times.
But the companies could encounter hurdles to gaining regulatory approval for the tie-up. A deal between T-Mobile and Sprint, who are the third largest and fourth largest wireless carriers in the United States by subscribers, previously faced opposition from antitrust regulators under President Barack Obama's administration.
Shares of Sprint and T-Mobile were essentially flat in extended trading.
Here's the full letter:
CNBC's Anjali Robins contributed to this report.