Washington (CNN)Sen. Tammy Duckworth is demanding more details about Russian bounties that were offered to the Taliban to kill US troops in Afghanistan and sent a new letter to Defense Secretary Mark Esper on Thursday voicing her frustration with the information provided by the Pentagon to date.

       The Illinois Democrat, who is a combat veteran and considered to be a top candidate to become former Vice President Joe Biden's running mate in the 2020 election, called a recent closed briefing by the Pentagon to the Senate Armed Services Committee on the issue of Russian bounties inadequate and claimed defense officials "were unprepared to respond to questions related to the disturbing public reports."

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       "It is unacceptable that to date, the Trump administration appears to be ignoring a matter of great importance to Gold Star Family members whose loved ones were killed while serving in Afghanistan: were any US troop casualties in Afghanistan connected with the alleged GRU bounty payments to Taliban-linked militants? Gold Star Families deserve an answer to this question," she wrote to Esper.

       Russian intelligence officers for the GRU, a military intelligence unit, offered money to Taliban militants in Afghanistan as a reward if they killed US or British troops there, a European intelligence official told CNN last month.

       On Thursday, Duckworth asked Esper to explain whether the Pentagon is conducting an investigation into "US troop casualties in Afghanistan to assess whether any US service member deaths are related to the alleged Russian bounty program."

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       She also inquired as to whether the Department of Defense is tracking any concurrent or completed intelligence community investigations into US troop casualties in Afghanistan and pushed Esper to commit to "disclosing the findings of all investigations on this subject with Gold Star Family members, Congress and the American people."

       The letter comes the same day Esper and Joint Chiefs Chairman Gen. Mark Milley are testifying before the House Armed Services Committee on Capitol Hill.

       It also comes amid an ongoing debate over the intelligence reports related to Russian bounties that have been reported by CNN and other media outlets. President Donald Trump has repeatedly claimed that he was not briefed on intelligence of this nature but US military and intelligence officials have not denied its existence.

       The top US general overseeing operations in the Middle East and Afghanistan said Tuesday that the intelligence concerning Russian operatives offering bounties to Taliban-linked militants was "very worrisome" but that the information wasn't solid enough to hold up in a court of law.

       Gen. Frank McKenzie, the commander of US Central Command, also told a small group of reporters while traveling to the region that he was not convinced that the Russian bounty program was directly responsible for the deaths of US personnel.

       Top US general says Russian bounty intelligence 'wasn't proved' but 'proved enough to worry me'

       "The intelligence wasn't proved to me. It was proved enough to worry me. It wasn't proved enough that I'd take it to a court of law. That's often true in battlefield intelligence," McKenzie said, according to a transcript provided by the Defense Department.

       His comments marked the first time a Pentagon official answered questions about the US intelligence that assessed there was an effort by a Russian military intelligence unit to pay the Taliban to kill US soldiers.

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