There was a press interview when the police released the news of his death, and one of the reporters asked if being hit on the head (apparently a glancing blow off the helmet from a thrown fire extinguisher) was connected to his death and the police spokesman said, “No.”.
Betcha that has gone down the memory hole.
As far as the popular story that is out there, that is based solely on some anonymous sources that said he was hit with a fire extinguisher, which led to his aneurysm. There is tape of a demonstrator throwing a fire extinguisher at police, but Officer Sicknick was not involved in that incident.
When Officer Sicknick came off duty, he commented that he had been maced, but said nothing about being hit with a fire extinguisher.
His family and official sources said whatever led up to is death was unknown.
The FBI got excited about the prospect of a murder charge but has backed off, and said merely it is “under investigation.”
Official sources have absolutely no motivation to clear up the record, the FBI is invested in the domestic terrorism angle, and loves to puff up (to the point of fabricating) crimes. The Dems certainly don't want to change the narrative, this offsets, and even trumps, the killing of the Trump supporter. No pub is going to risk the political firestorm if they question the story: (1) there is no upside,(2) they'll be demonized as being "disrespectful to the fallen officer hero by spreading conspiracy theories” (3) they actually have no problem with the government demonizing Trump supporters.