U.S. Sen. Amy Klobuchar on Friday fired off a letter to the CEOs of ATT and T-Mobile, pressing them to explain how ATT's $39 billion deal to buy T-Mobile won't lead to higher prices and hurt consumers.

Klobuchar sits on the Senate Judiciary subcommittee that on Wednesday will hear about the deal from the chief executives of the nation's largest wireless carriers.

In her letter, Klobuchar noted that wireless providers have resisted efforts to mandate pro-consumer measures.

"They always say, 'Don't worry, there's always competition in the marketplace that will take care of it,' " Klobuchar said in an interview Friday.

But if the deal is approved, ATT and Verizon Communications, the top two carriers, would control about 75 percent of the U.S. wireless market, creating a duopoly "and diminishing the possibility that the industry will act on its own to institute pro-consumer practices," Klobuchar wrote.

U.S. Sen. Al Franken, the state's junior member, sits on the same committee and also has concerns.

"Wall Street loves this type of media consolidation, but as we have seen in the past, consolidation like this is almost never good for consumers and will likely mean less choice and higher prices for wireless service," he said.

— Leslie Brooks Suzukamo