The FBI detailed its investigation in a memo sent to the private sector and others on Friday, the newspaper reported.
As rioters overpowered Capitol police, Sicknick was pepper-sprayed and hit in the head, his father told Reuters last week. Ambulance crews resuscitated him twice as he was rushed to a nearby Washington hospital. Sicknick died the next day.
The FBI did not immediately respond to a request for comment.
Sicknick’s death is being investigated as a homicide by the Washington Metropolitan Police. The FBI is assisting.
Sicknick was the fifth person to die after Trump supporters rioted at the US Capitol on Wednesday.
The Republican president had summoned them to Washington for a rally and urged them to fight as lawmakers were meeting to certify his November election loss to Democrat Joe Biden. The deaths included some rioters.
Fourteen other Capitol Police officers were injured in the riot, the FBI memo said, according to the New York Times.