(CNN) US Sen. Elizabeth Warren has written to US government agencies demanding answers after a CNN investigation revealed that American-made weapons in Yemen are being turned on the internationally recognized and US-backed government.

Last week, CNN revealed that military hardware supplied to US allies had been distributed in contravention of arms deals to militia groups, including separatists backed by the United Arab Emirates. The separatists are now using that hardware to fight the Saudi Arabia-supported forces of the internationally recognized government, who are also armed with US weapons.

"These unauthorized diversions of American military hardware to armed groups ... undermine US national security objectives in securing a political settlement to the conflict in Yemen, which has no military solution and remains one of the world's worst humanitarian crises," reads Warren's letter, which was sent Monday and is addressed to US Secretary of Defense Mark Esper and US Secretary of State Mike Pompeo.

This is the second time that Warren, a candidate for the 2020 Democratic presidential nomination, has written to the US agencies about arms transfers in Yemen following CNN reporting.

In February, a CNN investigation revealed that Saudi Arabia and the UAE had transferred American-made weapons to al Qaeda-linked fighters, hardline Salafi militias, and other factions waging war in Yemen, in violation of their agreements with the United States.

The weapons also made their way into the hands of Iranian-backed rebels battling the coalition for control of the country, exposing some of America's sensitive military technology to Tehran and potentially endangering the lives of US troops in other conflict zones.

Responding to the latest evidence published by CNN last week, a UAE official said: "There were no instances when US-made equipment was used without direct UAE oversight. Except for four vehicles that were captured by the enemy." The Saudi government has not responded to CNN's requests for comment on this issue.

Warren sent the agencies detailed questions about the supervision of US-made weapons but received no reply other than a "brief acknowledgment from the State Department," according to her letter.

"The latest report underscores the need for concrete answers to my initial inquiry, highlights the importance of preventing unauthorized access, unauthorized transfers, or other violations of end-user agreements by foreign governments, and raises legitimate questions about whether it is in America's interest to continue selling arms and other military hardware to the Saudi and UAE governments," wrote Warren.

CNN has reached out to the State Department and Pentagon for comment on Warren's letter.

Following CNN's initial reporting in February, the Pentagon said it had launched its own investigation into the unauthorized transfer of US weapons in Yemen. In response to CNN's new findings, Pentagon spokesperson Lt. Col. Carla Gleason said in September that the joint investigation by the State Department and Department of Defense remains "ongoing."

But more than half a year since the investigation was launched, the situation on the ground appears to have gotten worse.

Saudi Arabia has led a coalition, in close partnership with the UAE and including various militia groups, to fight the Iranian-backed Houthi rebels in Yemen since 2015. But, in a clear break with its Saudi partners, the UAE said in July that it was reducing its forces in the country, and fighting escalated between separatists and government forces on the ground in August. The UAE has since thrown its support behind the separatist movement.

In recent months there have been multiple efforts by US lawmakers to force President Donald Trump to end US financial and military backing for the war in Yemen.

Sen. Chris Murphy, a Democrat, authored an amendment to the annual US defense spending bill, which is currently being debated in Washington, that would cut off support for the Saudi-led coalition until the Secretary of Defense can certify that both the Saudis and Emiratis have stopped transferring US weapons to third parties in Yemen.

In June 2019 the Armed Conflict Location & Event Data Project (ACLED) reported that the total number of reported fatalities in Yemen is more than 91,000 since 2015.