Exactly one month since rioters stormed the U.S. Capitol on Jan. 6, fallen Capitol Police Officer Brian Sicknick’s official cause of death has not been released and no one has been charged with his death.

Metropolitan Police Department (MPD) Chief Robert J. Contee III confirmed at a news conference Thursday that the investigation into Sicknick’s death is ongoing, stressing that police continue to comb through video evidence, in the latest update provided by authorities.

Contee, speaking vaguely, also suggested Sicknick’s injuries may not have been immediately visible. "That determination is made by the Office of the Chief Medical Examiner, so MPD’s role in that is to make sure that the medical examiner has all of the evidence they need to make that determination," he said. "In this situation, with the Capitol insurrection, there were hundreds of videos and all of that kind of stuff -- that stuff is being gone through and funneled over to them."

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Contee said the medical examiner’s office will make a recommendation, "once they have a better understanding of what exactly to deal with," adding that, "sometimes, and not to speak for the doctors, when they are assessing individuals who may not have visible injuries -- that kind of thing -- you know they have to be very thorough in their efforts to make a determination in manner and cause of death."

Exactly how Sicknick died still remains a mystery to the public, as an official cause of death has not been released.

Media reports have been conflicting — unnamed law enforcement sources initially told outlets Sicknick was bludgeoned in the head by a fire extinguisher, while others speaking on condition of anonymity countered those claims, arguing there was no immediate evidence showing that Sicknick suffered any blunt force trauma.

As the autopsy results remain pending, investigators are also weighing the possibility that Sicknick could have died from exposure to a chemical irritant, such as bear mace or pepper spray. It also remains unclear whether Sicknick had any pre-existing conditions. 

The Office of the Chief Medical Examiner in Washington, D.C., told Fox News Thursday that its medical examiners "comply with the National Association of Medical Examiners’ (NAME) standard to determine the cause and manner of death within 90 days; however, for cases that are more complex it could be longer."

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"Therefore, when this information is available and the decedent’s next of kin has been notified, I will only provide you with the cause and manner of death," special assistant to Chief Medical Examiner Cheryle E. Adams said Thursday, without providing specifics. 

Lawmakers paid tribute to Sicknick earlier this week as his remains lay in honor in the Capitol Rotunda before a ceremonial send-off on the steps to Arlington National Cemetery, where he was interred Wednesday. 

In the only public statement issued by U.S. Capitol Police describing the circumstances surrounding his death, the department said Sicknick "passed away due to injuries sustained while on-duty." He died at the hospital at approximately 9:30 p.m. the evening of Jan. 7.

Sicknick "was responding to the riots" on Jan. 6 at the U.S. Capitol and "was injured while physically engaging with protesters," the statement said. "He returned to his division office and collapsed. He was taken to a local hospital where he succumbed to his injuries."

His eldest brother, Ken Sicknick, told ProPublica in an interview published Jan. 8 that Brian Sicknick had texted him on the night on Jan. 6 to tell him he had been pepper-sprayed but felt fine. He told the outlet his brother was dead by the next night, after suffering a stroke. 

Deceased Capitol Police Brian Sicknick will lie in the state in the Capitol before he is buried at Arlington National Cemetery. 

Deceased Capitol Police Brian Sicknick will lie in the state in the Capitol before he is buried at Arlington National Cemetery.  (U.S. Capitol Police)

"He texted me last night and said, ‘I got pepper-sprayed twice,’ and he was in good shape," Ken Sicknick told the outlet over the phone, as the family drove toward Washington, D.C., from New Jersey. "Apparently he collapsed in the Capitol and they resuscitated him using CPR."

The family later received word that Brian Sicknick had a blood clot and had had a stroke and a ventilator was keeping him alive, Ken Sicknick told ProPublica. Authorities have not verified his account, and Ken Sicknick has not returned a Fox News request for comment. 

Fox News also requested an update from U.S. Capitol Police, but a spokesperson declined to comment on an ongoing investigation. 

Two days after the insurrection, the Justice Department first launched an investigation into Sicknick’s death. 

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Acting Attorney General Jeffrey A. Rosen said in a statement on Jan. 8 that the FBI and D.C. Metropolitan Police Department will jointly investigate the case and the Justice Department "will spare no resources in investigating and holding accountable those responsible."

That day, North America’s Building Trades Unions (NABTU) also announced a $100,000 reward for information leading to the arrest of the person or persons responsible for Sicknick’s murder. Still, no suspect has been publicly identified or taken into custody.

Meanwhile, a criminal complaint filed by the Justice Department alleges that Robert Sanford, a retired firefighter from Pennsylvania, was captured on video hurling what appeared to be a fire extinguisher at a group of police officers outside the Capitol on Jan. 6. The officers were not named in the statement of facts filed in court, and officials, so far, have not linked Sanford to Sicknick’s death.

Video viewed by FBI investigators purportedly shows that the object thrown by Sanford "appears to strike one officer, who was wearing a helmet, in the head," court documents state. "The object then ricochets and strikes another officer, who was not wearing a helmet, in the head. The object then ricochets a third time and strikes a third officer, wearing a helmet, in the head."

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Sanford has been charged with unlawful entry into a restricted building or grounds, disorderly conduct and assault, and resisting or impeding certain officers in their performance of official duties. As of Thursday, he has not been charged with homicide.