White House officials believe Russia is preparing to falsify evidence to blame Ukrainian forces for the deadly blast at the Olenivka prison ahead of visits to the site by outside parties.
An administration official told CNN they expect Russia will falsify evidence, blame Ukrainian forces and even have "reason to believe that Russia would go so far as to make it appear that Ukrainian HIMARS were to blame before journalists arrive.”
The US supplied Ukraine with the HIMARS multiple launch rocket system earlier this year.
Ukrainian President Volodymyr Zelensky has said the attack on the prison, which is in separatist-held eastern Ukraine, was "a deliberate war crime by the Russians."
Russia has previously blamed Ukraine for the attack.
UN mission: United Nations Secretary-General António Guterres said in a Wednesday news conference that the UN is seeking to establish a fact-finding team to study the attack on the detention facility that resulted in at least 50 deaths and dozens of injuries of Ukrainian prisoners of war.
Russia and Ukraine both requested an investigation into the attack, Guterres told reporters in New York. He added that the terms of reference for the panel would need to be accepted by Russia and Ukraine before the fact-finding mission would begin.
Ukraine blames mercenary group: The Ukrainian Defense Ministry's Intelligence department claimed Wednesday that the detonation of the building where Ukrainian soldiers were held "was carried out by the fighters of the 'Wagner' military command center using a highly flammable substance, which led to the rapid spread of the fire in the premises."
Wagner is a private military contractor whose fighters have been involved in the Russian invasion of Ukraine, as well as other conflicts in Africa and the Middle East.
CNN is unable to verify the claim by the Ukrainian Defense Intelligence department, which provided no evidence to support its allegation.
What Russia is saying: The Russian government has rejected accusations from the White House that it is falsifying evidence.
“One thing can be said here, which is absolutely obvious and absolutely proven about what happened in Olenivka. People, namely the Ukrainian prisoners of war, who were kept there, they died at the hands of the Ukrainian military,” Kremlin spokesperson Dmitry Peskov told journalists in a call on Thursday.
“Ukraine killed its soldiers who were in captivity. Many wounded. Therefore, there is hard evidence here and there is nothing to hide,” Peskov said.
In Thursday’s call, Peskov said Russia had invited the UN and the ICRC to visit the site. However, in a statement issued on Wednesday, the ICRC said it still had not been given access to the site.
CNN's Uliana Pavlova contributed reporting to this post.