Plotkin said CFIUS and the Department of Homeland Security have been "jawboning" telecom companies for several years to make sure their equipment suppliers don't cause security concerns. In that vein, "CFIUS wants to have some oversight of the supply chain for Sprint," he said.

One risk arbitrageur tracking the deal attributed Dish's continued focus on the purported security risks to Dish chairman Charlie Ergen's determination not to give up any avenue that will help him get a deal done. "It's just a personality trait of Charlie Ergen," the source said.

Written by William McConnell in New York

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