The report is scheduled to be released Friday, The Washington Post reported. It was leaked Thursday to The New York Times.
Israel was urged to apologize to Turkey for the deaths of nine of its nationals and to pay compensation to the men's families, the report said.
U.N. Secretary-General Ban Ki-moon set up a special committee to investigate the May 2010 incident because of the discrepancies between Israeli and Turkish versions of what happened. The panel found Israel has produced no "satisfactory explanation" of why the killings occurred.
"Forensic evidence showing that most of the deceased were shot multiple times, including in the back, or at close range has not been adequately accounted for in the material presented by Israel," the report concluded.
Organizers said the flotilla was intended to bring humanitarian aid to Gaza. Israeli authorities said commandoes were acting self defense when they killed the nine activists.
A U.N. commission led by former New Zealand Prime Minister Geoffrey Palmer concluded in July the military operation was "premature" and the deaths "unacceptable," The Daily Telegraph reported.