A confidential source (paid snitch) for the FBI - proven reliable over several years - told them he had several conversations with the head of Burisma, Mykola Zlochesky. Burisma is the Ukraine natural gas company that employed Hunter Biden as a corporate director. The details of these conversations were reported to the FBI and recorded by them on a form FD-1023.
Today that form was released by Sen. Grassley and it shows the source claimed he was told Burisma paid $5 million each to Hunter and Joe Biden for their role in getting a Ukrainian prosecutor - Viktor Shokin - fired. He was investigating corruption at Burisma. Video of Joe Biden bragging about getting the prosecutor fired is available at YouTube and perhaps elsewhere.
The Daily Mail (U.K.) reports the FD-1023 says Burisma's CEO claimed that although Hunter "'was stupid' and the CEO's dog was smarter, he was needed on the board 'so everything will be okay'" The CEO also told the source he had text messages and 17 recordings to substantiate his claims. For additional commentary and a copy of the actual document, see John Hinderaker's column at Power Line.
The FBI has had these allegations since before the election in 2020 and they also had the Hunter Biden laptop. If an investigation of the contents was undertaken, no evidence thereof has surfaced.
FBI inaction in this matter doesn't pass the smell test.