The Federal Communications Commission (FCC) revealed AT&T as the leading bidder in the record-breaking AWS-3 auction but is casting a sceptical eye over some $3.3 billion of discounts claimed by second-placed Dish Network.

AT&T tabled winning bids worth $18 billion, accounting for some 40 per cent of the $44.9 billion sale of radio frequencies, which closed last week.

However, controversy threatens what was otherwise a highly lucrative contest for the US regulator.

The FCC is reviewing the $3.3 billion in discounts claimed by Dish on bids it made via a pair of partnerships — Northstar Wireless and SNR Wireless Licensee Co.

FCC’s auctions are designed to encourage bids from small firms, in order to spread some diversity in network ownership. Both Northstar and SNR claim less than $15 million in annual revenue.

Dish’s partners in the two firms include investment firm BlackRock, employees of private-equity firm Madison Dearborn Partners and Doyon Ltd, a native Alaskan investment fund.

Ajit Pai, one of the FCC’s five commissioners, plans to raise the issue with chairman Tom Wheeler, claimed the Wall Street Journal, seeking an investigation of Dish’s bid tactics.

It wouldn’t be the first time such a controversy has arisen in a FCC auction.

Dish’s winning bids totaled $13.3 billion, putting it ahead of Verizon Wireless which ponied up a total of $10.4 billion.

T-Mobile US had total winning bids of $1.8 billion in the AWS-3 contest. Sprint, the final member of the mobile industry’s big four, was not among the bidders.

Once the dust has cleared, Dish’s intentions for its stash of spectrum will be a source of speculation.

In the past, it has been thought that the satellite firm intended to sell its spectrum to Verizon Wireless. Dish has also been linked with the purchase of T-Mobile US, or even building its own wireless business.