The US House can ask and/or subpoena anyone in Afghanistan with knowledge of the Russian assassination for money directive to testify and/or provide evidence of the program. I doubt the program was word of mouth. The US House could accept that testimony and/or evidence in or/and out of the sovereign country of the USA. An embassy perhaps. I remind one of the reasons the USA knows about this program as a matter of fact is because people in Afghanistan were willing to speak about the Russian money.