The only investigation was done by a UK commission, so obviously they are being very diplomatic towards their own government that was part of the war crime coalition in Libya. Basically, they conclude that the US-armed rebels were the ones causing most of the massacres and the NATO coalition used their own crimes as a pretext to invade.

This policy was not informed by accurate intelligence. In particular, the Government failed to identify that the threat to civilians was overstated and that the rebels included a significant Islamist element. By the summer of 2011, the limited intervention to protect civilians had drifted into an opportunist policy of regime change. That policy was not underpinned by a strategy to support and shape post-Gaddafi Libya. The result was political and economic collapse, inter-militia and inter-tribal warfare, humanitarian and migrant crises, widespread human rights violations, the spread of Gaddafi regime weapons across the region and the growth of ISIL in North Africa. Through his decision making in the National Security Council, former Prime Minister David Cameron was ultimately responsible for the failure to develop a coherent Libya strategy.

https://publications.parliament.uk/pa/cm201617/cmselect/cmfaff/119/119.pdf

https://www.salon.com/2016/09/16/u-k-parliament-report-details-how-natos-2011-war-in-libya-was-based-on-lies/