The Federal Communications Commission said Tuesday it needs more time to review the planned merger of T-Mobile US Inc. and Sprint Corp.

The third- and fourth-largest mobile carriers in the U.S agreed in April to merge in a$26 billion deal and were given a 180-day period by the FCC for the merger to be reviewed. The two companies have said the alliance would give them an edge over competitors to build a 5G network.

The companies had unsuccessfully tried to merge in 2014 and 2017, with regulators stopping the first attempt and disagreement over how much control the companies would have nixing the deal on the second attempt.

The pause comes on the same day both AT&T Inc. and Verizon Communications Inc. announced a rollout of 5G internet connectivity in certain U.S. cities.

As the FCC’s “shot clock” hit day 55, the FCC said, “additional time is necessary to allow for thorough staff and third-party review of newly submitted and anticipated modeling.” This is largely because it needs more time to review a revised network engineering model from the two companies.

The FCC said the model is “significantly larger and more complex than the engineering submissions already in the record.” On top of that, it said, it also requires some extra time to go over a business model named “Build 9,” which will provide the financial basis for the projected new network buildout. The commission said it only received information pertaining to Build 9 on Sept. 5.

“The clock will remain stopped until the applicants have completed the record on which they intend to rely, and a reasonable period of time has passed for staff and third-party review,” said the FCC. The commission will then decide if the deadline needs to be extended.

T-Mobile and Sprint responded, saying they understood the need to stop the clock. “The additional review time is common to FCC merger reviews and we recently supplied a large amount of data to the FCC that they want sufficient time to assess,” the companies said in a statement. “We are confident that this transaction is pro-competitive, good for the country and good for American consumers, and we look forward to continuing to work with the FCC as they evaluate our plans.”

Image: Marco Verch/Flickr