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(CNN) —  

The US State Department and Pentagon are sending teams to the UAE and Saudi Arabia to investigate CNN’s findings that American-made weaponry has been transferred to rebel fighters and separatist militia in Yemen.

In a letter obtained by CNN, the State Department says the “continued insufficient responses” of both US allies have delayed the probe, which was launched in response to CNN reporting earlier this year.

CNN’s ongoing investigation has found that American-made armored vehicles (known as Mine-Resistant Ambush Protected vehicles, or MRAPs) which were sold to the UAE and Saudi Arabia have been transferred – in violation of their agreements with Washington – to groups including al Qaeda-linked fighters, Iranian-backed rebels and separatist militias.

These factions have since turned them against the internationally-recognized Yemeni government forces which the US supports.

Following CNN’s initial reporting in February, the State Department said it had launched a joint investigation with the Pentagon into the unauthorized transfer of weapons in Yemen.

This latest letter from the State Department’s Bureau of Legislative Affairs is the first substantive update on the progress of the investigation since then.

It says that a joint State-Pentagon visit was carried out to the UAE in September to verify what had happened to the MRAPs the US supplied. The letter adds that another similar “oversight visit” to Saudi Arabia is planned for November, after which “the Department expects to have a full account of the circumstances related to the disposition of this equipment and any potential violation of the agreements.”

Related Article: Sold to an ally, lost to an enemy

The State Department letter was sent to presidential candidate Senator Elizabeth Warren, who has contacted the administration at least twice this year demanding answers about America’s continued supply of weaponry to the devastating conflict in Yemen.

In response to this latest update, Warren said she is “troubled by the apparent lack of full cooperation in this process by Saudi Arabia and the United Arab Emirates, which calls into question whether it is in America’s interest to continue selling arms and other military hardware to these governments.”

The State Department has not responded to requests for comment on this letter.

CNN has contacted both the UAE and Saudi governments for comment on the State Department letter.

A spokesman for the Saudi-led coalition, Col. Turki Al-Maliki, responded to the State Department’s claim they had received “continued insufficient responses” from Saudi Arabia, saying that “all required information (is) available.”

Earlier this month, Al-Maliki told CNN that “all military equipment is used by Saudi forces in accordance with terms and conditions of Foreign Military Sales (FMS) adopted by the US government and in pursuance of the Arms Export Control Act.”

In October a senior Emirati 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.”

Earlier this month a peace deal was signed between the UAE-backed separatists and Saudi-backed Yemeni government forces, which has gone some way to de-escalating the fighting in Yemen. But as of October 31 the war there has killed more than 100,000 people since 2015, according to the Armed Conflict Location & Event Data Project.

Three-quarters of the country’s population is in need of humanitarian assistance, according to the United Nations.

CNN’s Sarah El Sirgany contributed to this report.