The U.S. Justice Department extended its review of AT&T Inc.’s proposed $39 billion purchase of T-Mobile USA Inc., asking the companies for more information, according to two people familiar with the situation.

The department’s antitrust division also sent so-called civil investigative demands, which are like subpoenas, to competitors of the two companies, seeking data on how the transaction might affect their businesses, according to the people, who declined to be identified because the review is confidential.

The second round of requests to AT&T and T-Mobile will allow the Justice Department to extend its review of the transaction indefinitely.

“This confirms that the government is going to do a deep dive into the transaction to understand what the implications are,” said Rebecca Arbogast, a Washington-based analyst with Stifel Nicolaus & Co.

Michael Balmoris, a Washington-based spokesman for AT&T, and Hernan Daguerre, a T-Mobile spokesman, both declined to comment.

Gina Talamona, a spokeswoman for the Justice Department, said she doesn’t comment on ongoing investigations.

The proposed acquisition, announced March 20, would combine the second- and fourth-largest U.S. wireless operators to create the nation’s largest. If approved by the FCC and the Justice Department, the deal would leave three major wireless providers in the U.S., including Sprint Nextel Corp. and Verizon Wireless, which is co-owned by Verizon Communications Inc. and Vodafone Group Plc.

The acquisition would push AT&T past its largest rival, Verizon Wireless, to become the biggest U.S. mobile-phone carrier. AT&T and T-Mobile combined have 39 percent of the market, according to research firm EMarketer Inc.

Jonathan Chaplin, an analyst at Credit Suisse Group AG in New York, said when the deal was announced that he had never seen a transaction with such high regulatory risk. Antitrust experts have estimated it could take as much as a year before the transactions are approved.

It is standard procedure under U.S. antitrust law for the agency reviewing the transaction to issue a second request if it needs more time to review the possible impact of the transaction on the market.