Report of the Select Committee on Intelligence, United States Senate on Russian Active Measures Campaigns and Interference in the 2016 U.S. Election; Volume 3: U.S. Government Response to Russian Activities (Redacted) [open pdf - 4MB]
From the Introduction: "Senior U.S. Government officials in both the Executive and Legislative Branches believed they were in uncharted territory in the second half of 2016. They became aware of aspects of Russian interference in U.S. elections over the summer and fall, but these officials had incomplete information on the scope of the threat. In the fall, the Obama administration responded with several warnings to Moscow, but tempered its response over concerns about appearing to act politically on behalf of one candidate, undermining public confidence in the election, and provoking additional Russian actions. Further, administration officials' options were limited by incomplete information about the threat and having a narrow slate of response options from which to draw. After the election, President Obama took action to punish Moscow for its interference, including instituting sanctions, expelling Russian government personnel, and shuttering Russian diplomatic facilities inside the United States."
S. Rept. II6-XX; Senate Report 116-XX
U.S. Senate Select Committee on Intelligence: https://www.intelligence.senate.gov/