The U.S. Justice Department has filed suit to block AT&T’s planned $39 billion deal to buy a rival mobile phone company called T-Mobile.

AT&T is the second-largest mobile phone company in the country, while T-Mobile ranks fourth.

In court papers filed Wednesday, Justice officials said the deal could hurt competition and raise prices for consumers.

If regulators reject the transaction, AT&T would pay the struggling T-Mobile $3 billion in cash and billions more in rights to use certain parts of the radio spectrum.

T-Mobile is a unit of German-based Deutsche Telekom.