SHANGHAI — With an advanced screen, a special artificial-intelligence microchip and an eye-popping price, the newest smartphone from Huawei Technologies was meant to show Americans what China can do with technology.

Instead, Huawei’s push to sell the phone in the United States has suddenly lost a powerful backer — and the push has attracted some unwanted scrutiny from Washington.

AT&T walked away from a deal to sell the Huawei smartphone, the Mate 10, to customers in the United States just before the partnership was set to be unveiled, two people familiar with the plans said on Tuesday, speaking on the condition of anonymity because the discussions were not public. The Wall Street Journal reported earlier that AT&T had changed plans.

The reasons that led to AT&T’s shift were not entirely clear. But last month, a group of lawmakers wrote a letter to the Federal Communications Commission expressing misgivings about a potential deal between Huawei and an unnamed American telecommunications company to sell its consumer products in the United States. It cited longstanding concerns among some lawmakers about what they said were Huawei’s ties to the Chinese government.