(CNN) Elizabeth Warren wants to know how American weapons ended up in the wrong hands in Yemen.

On Thursday, the US senator and Democratic presidential candidate issued a letter with 13 questions about the suspected retransfer of arms from US allies to third-party actors in the wartorn country.

Addressed to acting Defense Secretary Patrick Shanahan and Secretary of State Mike Pompeo, the letter cited evidence of arms deal violations uncovered by a CNN investigation

"Ensuring that foreign governments do not divert American weapons to third parties is an important and necessary way to hold allies and partners accountable and protect U.S. national security," Warren wrote.

Saudi Arabia and the UAE are transferring US-made weapons to other groups

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