The U.S. Justice Department has charged four Singaporeans and an Iranian with smuggling American-made radio devices to Iran, where the items allegedly were used to make remote-controlled bombs that have killed U.S. troops in Iraq.

In a statement Tuesday, federal prosecutors said Singaporean authorities arrested the four Singapore citizens the previous day in connection with the case, while the Iranian suspect, Hossein Larijani, remains at large.

The Justice Department indictment says the defendants bought 6,000 radio control devices from a company based in the U.S. state of Minnesota, shipped the items to Singapore between 2007 to 2008, and forwarded them to Iran in violation of U.S. export controls.

Prosecutors say 16 of the devices were recovered in unexploded roadside bombs in Iraq between 2008 and 2010. Such bombs caused most of the American combat casualties in Iraq for several years after the 2003 U.S.-led invasion. Washington has long accused Iran of arming Iraqi Shi'ite militants responsible for the roadside bombings and other attacks in Iraq, a charge Tehran denies.

U.S. assistant attorney general for national security Lisa Monaco said the radio device smuggling case “underscores the continuing threat posted by Iranian procurement networks seeking to obtain U.S. technology through fraud and the importance of safeguarding that technology.”

The indictment filed in a Washington court includes charges of conspiracy to defraud the United States, smuggling, illegal export of U.S. products, false statements and obstruction of justice.

The four Singaporean defendants were identified as Wong Yuh Lan, Lim Yong Nam, Lim Kow Seng and Hia Soo Gan Benson.