Just when T-Mobile (TMUS) and Sprint (S) thought their merger was moving forward, following the approval by the Federal Communications Commission (FCC) on Monday, the U.S. Justice Department (DOJ) is looking to block it. The antitrust division staff has recommended that the DOJ file a lawsuit that could prevent the mobile phone carriers’ acquisition from getting the go-ahead, two sources close to the matter told Reuters.

T-Mobile is looking to acquire Sprint for $26 billion, but the companies have faced an uphill battle as they look to finalize the deal. Now, the decision lies with the DOJ department’s antitrust division which is led by Makan Delrahim, who is “leaning against the merger,” as the DOJ has concerns that the acquisition would reduce competition in the market, Bloomberg reported.

The DOJ is expected to come forward with a decision about the T-Mobile-Sprint merger in a month; the two sources told Reuters. The news of the DOJ’s push back comes after Monday’s decision by the FCC to approve the deal between the two companies. Sprint was, however, required to sell its Boost Mobile business in order to finalize the acquisition. In addition, the companies must implement 5G and increase their mobile speeds – something the two carriers had expressed they are planning.

The DOJ may decide to block the deal with an antitrust lawsuit, similar to what was attempted with the AT&T-Time Warner merger. While Delrahim lost that lawsuit, he has not reportedly come to a decision on whether to file a complaint against T-Mobile and Sprint to date.

Shares of T-Mobile stock were down 0.69 percent as of 2:45 p.m. ET on Wednesday while shares of Sprint stock were down 7.13 percent at the same time.