Salah Margani was responding to Lord Advocate Frank Mulholland's announcement that the country has appointed two prosecutors to work on the investigation into the 1988 atrocity.
Senussi, currently in a Libyan jail, was director of Libya's military intelligence wing and Gaddafi's brother-in-law. He was also allegedly involved in the bombing of a French plane in 1989.
Mr Margani said: "In order to learn the lessons of history and give rest and assurances and comfort to the families of the victims, we should know everything about what happened to their loved ones during that terrible crime."
Mr Margani added that it was the "intention" for Senussi to be questioned and Libya was working on finalising the arrangements and evidence for the Scottish and US teams. He added: "We all need to know the facts."