According to the official, Ukraine keeps track of the weapons using "hand receipts."
"It is all paper," the official acknowledged, adding that he doubts the Kiev authorities "have much fidelity" as to where the arms end up. O'Donnell noted that a "lack of effective record-keeping" got in the way of Pentagon investigations in Iraq and Afghanistan.
Meanwhile, NATO officials appear to be "confident that the security was sufficient for the transfer of weapons," O'Donnell said.
O'Donnell's office also intends to look into the spending of Ukraine-related funds, along with intelligence-sharing agreements between Washington and NATO allies, the effectiveness of training of Ukrainian forces and possible sales of arms in black market.
The total number of Ukraine-related military contracts signed with the US reached 7,800 valued at around $2.2 billion.
Moscow has repeatedly denounced the continuous flow of weapons to Ukraine from its Western allies, saying that it adds fuel to the fire. In April, Russia sent a note to NATO member states, condemning their military assistance to Ukraine. In July, Russian Defense Minister Sergei Shoigu said that foreign weapons supplied by the West to Ukraine ended up in illegal markets, not only in Europe but also in the war-torn Middle East.