The top U.S. commander in Afghanistan says in a new interview that he has not seen evidence to corroborate claims that Russian officials placed bounties on U.S. troops stationed in the Middle East.

Speaking with NBC News, Gen. Frank McKenzie said that reports in The New York Times and other media outlets indicating that U.S. intelligence officials were investigating reports that Russia was offering bounties to Taliban fighters for targeting U.S. service members had not been corroborated by evidence he had seen personally.

“It just has not been proved to a level of certainty that satisfies me,” Gen. McKenzie told NBC News, adding, “We continue to look for that evidence. I just haven’t seen it yet. But … it’s not a closed issue.”

The general added in the interview that he had directed officials under his command to investigate the allegations, which have been seized upon by critics of the president as evidence of President Trump‘s reluctance to criticize or confront Russian officials on foreign policy issues.

Democrats and some Republicans on Capitol Hill as well as Trump’s 2020 opponent, former Vice President Joe Biden (D), have demanded answers from the administration for months over the news reports, which indicated that some U.S. intelligence officials believed a Russian military unit was behind an offer to provide funds to Taliban insurgents in exchange for targeting U.S. and coalition forces in Afghanistan.

“I found what they presented to me very concerning, very worrisome. I just couldn’t see the final connection, so I sent my guys back and said, look, keep digging. So we have continued to dig and look because this involves potential threats to U.S. forces, it’s open,” said the general. “I just haven’t seen anything that closes that gap yet.”

Joint Chiefs of Staff Chairman Gen. Mark Milley, previously vowed to investigate the claims first raised in the Times, though top officials in the Trump administration have insisted for months that the intelligence was uncorroborated.

“I’ve got three tours in Afghanistan and multiple tours in a lot of other places, and I’ve buried a lot of people in Arlington National Cemetery, so I am committed to the nth degree to protect our force,” Milley told the House Armed Services Committee in July.

“Units of people are and were informed and will remain informed,” Milley continued at the time. “We’re going to get to the bottom of all that, but I can assure the families that force protection of our force, not only for me but for every commander all the way down the line, that’s the No. 1 priority for every one of us.”