(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
CNN's report, published earlier this month, revealed that Saudi Arabia and the United Arab Emirates had transferred US-made weapons to al Qaeda-linked fighters, hardline Salafi militias, and other groups on the ground.
The weapons have also made their way into the hands of Iranian-backed rebels, exposing some of America's sensitive military technology to Tehran and potentially endangering the lives of US troops in other war zones.
By handing off this military equipment to third parties, the Saudi-led coalition is breaking the terms of its arms sales with the US, according to the Department of Defense.
The senator from Massachusetts also cited public testimony by Gen. Joseph Votel, the top US military commander in the Middle East, who told a Senate hearing this month that the Department of Defense has mechanisms to monitor and enforce end user restrictions and "ha[s] not authorized Saudi Arabia or the Emirates to retransfer any of this equipment to other parties on the ground in Yemen."
Votel, the head of Central Command (CENTCOM), also said that the military has to "look more closely at the allegations" in CNN's report.
Warren's questions about US oversight of weapons
"If this report is true, it raises serious concerns that Saudi Arabia, the UAE, and other governments may have violated their end user agreements with the United States by diverting American weapons to terrorists and other violent extremists without prior authorization from the U.S. government," Warren added.
As a member of Senate Armed Services, Warren has previously questioned the Pentagon about whether they tracked US fuel and bombs used in Saudi airstrikes that killed civilians in Yemen.
To address her concerns about a potential national security threat, Warren asked Shanahan and Pompeo to explain US oversight of weapons sold to the Saudi-led coalition and other governments, its ability to track them and enforce restrictions on their retransfer.
February 9 was the deadline for US Secretary of State Mike Pompeo to re-certify to Congress that Saudi Arabia and the United Arab Emirates are taking adequate measures to avoid harming civilians, thereby allowing US-provided aerial refueling services to the coalition to continue. The State Department did not respond to CNN's request for comment on the certification deadline.
A war powers resolution that would require US President Trump to cut off military support to Saudi Arabia in Yemen is headed to the Senate after it was passed by the US House of Representatives last week.
Previous CNN investigations established that US-made weapons were used in a series of deadly Saudi coalition attacks that killed dozens of civilians, many of them children.
The Saudi coalition did not respond to multiple requests for comment. A senior UAE official denied "in no uncertain terms" that it had violated its end-use agreements with the US.
After CNN presented its findings, a US defense official confirmed that there was an ongoing investigation into the issue.
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