(CNN)Iran may use detained Americans who were not part of last month’s prisoner swap as leverage in future dealings with the United States, America’s intelligence chief said Tuesday.

Director of National Intelligence James Clapper’s testimony comes after a coalition of Iranian-American groups sent a letter last week to Secretary of State John Kerry, urging the U.S. to do more to secure the release of American businessman Siamak Namazi, who is still being held by Iran.

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Tehran released five U.S. citizens last month, including Washington Post reporter Jason Rezaian, Christian pastor Saeed Abedini and Marine veteran Amir Hekmati.

Iran however, “might attempt to use any additional U.S. citizens” held in Iran “as bargaining chips for U.S. concessions,” Clapper wrote in a worldwide threat summary submitted to Congress on Tuesday.

Clapper added that Americans detained upon entering Iran, like the 10 U.S. sailors held overnightlast month after drifting into Iranian territorial waters, may be used as bargaining pieces as well for Iran “to achieve financial or political concessions in line with their strategic intentions.”

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Namazi, who was arrested in Iran in October, was not set free as part of last month’s prisoner swap deal and still remains in Iran’s notorious Evin prison. He is the only other American known to be detained in Iran. The whereabouts of former FBI agent Robert Levinson, who went missing in Iran in 2007, are unknown. Iran has denied holding Levinson.

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In a letter on Friday, the five Iranian-American groups wrote that “one man has been left behind, an American who has worked tirelessly to build bridges between Iran and the United States: Siamak Namazi.”

source cnn.com