After the week began with good news for T-Mobile and Sprint's merger, it's now taken a U-turn.

The U.S. Justice Department antitrust division has recommended that the T-Mobile-Sprint merger be blocked. That's according to Reuters sources who explain that the DOJ staff worry that if T-Mobile and Sprint's deal is approved, T-Mo will not aggressively work to cut prices, improve service, and steal customers from Verizon and AT&T.

The Justice Department is expected to make its final decision on the merger in about a month.

This is a big deal for T-Mobile and Sprint, who were likely feeling positive about their deal's odds after FCC Chairman Ajit Pai and some Commissioners came out in support of the merger earlier this week. T-Mobile and Sprint made commitments that it promised to follow through on if the deal is completed, including the sale of Boost Mobile. But now the deal is in doubt with the news that the Justice Department's antitrust division has recommended it be blocked.