In this particular case, CrowdStrike saw circumstantial evidence of data exfiltration from the DNC network. As a reference point circumstantial evidence is the type of evidence such as DNA analysis or fingerprints that are fully admissible in courts.
“Counsel just reminded me that, as it relates to the DNC’ we have indicators that data was exfiltrated. We did not have concrete evidence that data was exfiltrated from the DNC, but we have indicators that it was exfiltrated.’
and circumstantial evidence that data was taken as he states on page 75 ”so there is circumstantial evidence that it was taken”
https://www.crowdstrike.com/blog/bears-midst-intrusion-democratic-national-committee/
Crowdstrike defends it's work with it's own blog.
"A lot of — outside of any political organization, companies, most corporations, they often would use these third party contractors, who they hired through their own counsel, and maximize the control from the point of view of the victim.”
Outside of any political organization .... Lol.
"We’ve provided all forensic evidence and analysis to the FBI related to the DNC investigation as requested. We have never declined any request for information from the FBI related to this investigation, and there are no pending requests for information by the FBI."
The FBI: 'That's not exactly true'
According to the Senate Intelligence Committee, CrowdStrike delivered a draft report to the FBI on Aug. 31, 2016 that an unidentified FBI official described as "heavily redacted." James Trainor, then-assistant director of the FBI's Cyber Division, told the committee that he was "frustrated" with the CrowdStrike report and "doubted its completeness" because "outside counsel" – i.e. Sussmann – "had reviewed it." According to Trainor, the DNC's cooperation was "moderate" overall and "slow and laborious in many respects." Trainor singled out the fact that Perkins Coie – and specifically, Sussmann – "scrubbed" the CrowdStrike information before it was delivered to the FBI, resulting in a "stripped-down version" that was "not optimal."