The Justice Department urged the U.S. telecommunications regulator on Tuesday to "defer" action on its investigation of Japan's Softbank Corp's takeover of Sprint Nextel, according to an FCC filling.

Softbank and Sprint agreed last year that Softbank Corp would pay $20 billion for control of Sprint Nextel Corp, giving the Japanese firm entry into the U.S. market and Sprint, the No. 3 U.S. carrier, much-needed cash.

The Justice Department, along with the Department of Homeland Security, put a letter on the Federal Communications Commission's docket that they were looking at the deal.

"We therefore request that the commission defer action until such time as the agencies notify the commission of the completion of their review," the letter said.

The FCC had no comment on the letter.

The review was likely aimed at ensuring that Softbank would continue to work with U.S. law enforcement to create wire taps when there was a court order, said James Lewis, a cybersecurity expert with the Center for Strategic and International Affairs.

The DHS concern is likely focused on ensuring the security of critical infrastructure, he said.He said there was minimal risk to the deal.