According to Bloomberg, the US Department of Justice has just announced that it will submit a deferment request for the Sprint / Softbank merger to the FCC. Telecom acquisitions in the United States require approval from both the DoJ and the FCC, with the former agency focusing largely on potential anti-trust issues. Softbank officially announced the buyout in October.
It's not exactly a big surprise that the DoJ is looking at this deal cautiously. After the failure of the AT&T / T-Mobile merger for potential anti-trust and consumer protection concerns, the DoJ likely feels compelled to investigate a foreign takeover of a major US telecom with extra care.