From the DOJ:
“The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation. The department also is providing assistance to and cooperating with the state officials in their investigation into the incident. With all federal civil rights crimes, the government must prove beyond a reasonable doubt that a person acted intentionally and with the specific intent to do something which the law forbids – the highest level of intent in criminal law.
“Negligence, recklessness, mistakes and accidents are not prosecutable under the federal criminal civil rights laws.”