T-Mobile US Inc.'s plan to buy Sprint Corp. requires close scrutiny over Sprint's owner SoftBank Group Corp. and its ties to the Chinese telecom firm Huawei Technologies Co., say U.S. lawmakers, according to a Bloomberg News report. Ahead of the July 4 trading holiday in the U.S., Sprint stock is down 0.2% in after hours trading and T-Mobile shares are flat. Lawmakers are circulating a draft letter to Treasury Secretary Steve Mnuchin, who is set to head the national security review of the T-Mobile-Sprint deal, according to Bloomberg. The committee on Foreign Investment in the U.S., or CFIUS, needs to approve the $26.5 billion merger. President Donald Trump could block the merger based on the CFIUS recommendation. SoftBank's work with Huawei includes a joint effort to demonstrate a new generation of wireless technology, among other things, according to Bloomberg. T-Mobile stock has lost 6.9% this year, as the benchmark S&P 500 index has gained 2%; Sprint shares fell 8.1%.