(CNN)Investigators are struggling to build a federal murder case regarding fallen US Capitol Police officer Brian Sicknick, vexed by a lack of evidence that could prove someone caused his death as he defended the Capitol during last month's insurrection.

Authorities have reviewed video and photographs that show Sicknick engaging with rioters amid the siege but have yet to identify a moment in which he suffered his fatal injuries, law enforcement officials familiar with the matter said.

In Sicknick's case, it's still not known publicly what caused him to collapse the night of the insurrection. Findings from a medical examiner's review have not yet been released and authorities have not made any announcements about that ongoing process.

According to one law enforcement official, medical examiners did not find signs that the officer sustained any blunt force trauma, so investigators believe that early reports that he was fatally struck by a fire extinguisher are not true.