The four Javelin anti-tank missiles, which cost more than $170,000 each, had ended up bolstering the arsenal of Gen. Khalifa Hifter, whose forces are waging a military campaign to take over Libya and overthrow a government the United States supports.
Markings on the missiles' shipping containers indicate that they were originally sold to the United Arab Emirates, an important American partner, in 2008.
If the Emirates transferred the weapons to General Hifter, it would likely violate the sales agreement with the United States as well as a United Nations arms embargo.
Officials at the State Department and Defense Department said Friday they had opened investigations into how the weapons ended up on the Libyan battlefield.
"We take all allegations of misuse of U.S. origin defense articles very seriously," a State Department official said in a statement. "We are aware of these reports and are seeking additional information. We expect all recipients of U.S. origin defense equipment to abide by their end-use obligations."