A senior law enforcement official tells Fox News that investigators are "not ruling chemical irritants out" as a potential factor in the death of Capitol Police Officer Brian Sicknick during the Jan. 6 riot.

The source noted that the investigation remains open and that officials are "looking at everything." 

Capitol Police officer Brian Sicknick ((U.S. Capitol Police))

Sicknick, 42, died in the hospital just one day after protecting lawmakers during the Jan. 6 riot at the U.S. Capitol.  

Law enforcement sources confirmed to Fox News that an autopsy has been performed on Sicknick, but that a cause of death and other information from the medical examination have not been revealed. Fox News was told immediately after Sicknick’s death that authorities would treat the probe as a homicide investigation. 

Sicknick’s remains were cremated before he laid in honor in the Capitol Rotunda last week, as dozens of law enforcement officials, congressional leaders, President Biden and first lady Jill Biden paid their respects. 

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In a letter on Thursday, House Speaker Nancy Pelosi, D-Calif., asked for members to sign on to cosponsor legislation awarding the Congressional Gold Medal to the Capitol Police.

"The outstanding heroism and patriotism of our heroes deserve and demand our deepest appreciation, which is why I am honored to introduce legislation to pay tribute to the Capitol Police and other law enforcement personnel who protected the U.S. Capitol on January 6 with the Congressional Gold Medal: the highest honor that the Congress can bestow," Pelosi wrote. "The service of the Capitol Police force that day brings honor to our Democracy, and their accepting this Gold Medal will bring luster to this award."

Pelosi added that we must "never forget the sacrifice of Officer Brian Sicknick, Officer Howard Liebengood, MPD Officer Jeffrey Smith and the more than 140 law enforcement officers who sustained physical injuries, or the courage of heroes such as Officer Eugene Goodman."  

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The latest on the Sicknick investigation comes as FBI special agents and members of the agency's Joint Terrorism Task Force arrested three individuals in Missouri and Kansas on federal charges related to the violence on the U.S. Capitol. 

William Chrestman, of Olathe, Kan., was arrested Thursday on charges of conspiracy, civil disorder, obstruction of an official proceeding, threatening to assault a federal officer, knowingly entering or remaining in a restricted building or grounds without lawful authority, and disorderly conduct.

Christopher Kuehn, of Olathe, was also arrested on federal charges of conspiracy, civil disorder, obstruction of an official proceeding, knowingly entering or remaining in a restricted building or grounds without lawful authority, and disorderly conduct.

Louis Colon, of Blue Springs, Mo., was arrested on federal charges of conspiracy, civil disorder, obstruction of an official proceeding, knowingly entering or remaining in any restricted building or grounds without lawful authority, and disorderly conduct on Capitol grounds.

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According to the criminal complaints, the trio "did advance together as they unlawfully entered the the grounds of the U.S. Capitol and the Capitol building itself" and proceeded to "act in concert to prevent law enforcement officers from controlling the crowd by obstructing metal barriers that had been deployed to prevent the crowd's further advancement into other areas of the Capitol."

William Chrestman (Photo courtesy of Department of Justice)

In images captured from the Jan. 6 riot, Chrestman appears to be wearing a black baseball hat, black glasses, a black sweatshirt, black boots, tan gloves, green and tan camoflauge pants, and a green tactical vest. He also was carrying what appeared to be a wooden club or axe handle disguised as a flag. 

Christopher Kuehn (Photo courtesy of Department of Justice)

Kuehn appears to be wearing a tan helmet with orange flourescent tape affixed to the top and back, a black and tan face mask, a black jacket, tan pants, orange tinted sunglasses and a black backpack with two rolls of orange hanging from the back. 

Louis Colon (Photo courtesy of Department of Justice)

Colon appears to be wearing a black helmet with orange flourescent tape affixed to the front and back, an American flag pattern face mask, an orange hooded sweatshirt, black sunglasses, black pants, black gloves, and a black backpack with a red bandana tied to the back of the backpack. 

Chrestman, Kuehn, and Colon marching among Proud Boys (Photo courtesy of Department of Justice)

Cell site data lawfully obtained by federal officials  indicates that all three men were present in or around the U.S. Capitol on Jan. 6 and other videos and images appear to show the men marching alongside Proud Boy organizers Joseph Biggs and Ethan Nordean.  

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At one point during the violent siege, Chrestman yelled at police officers "you shoot and I'll take your f***ing a** out!"

Upon advancing toward the front plaza of the Capitol, Chrestman appeared to take off his black helment and donned what appears to be a respirator. Video footage also shows Chrestman addressing the crowd shouting "Whose house is this?," to which the crowd replied "Our house!." Chrestman then shouted "Do you want your house back?" to whcih the crowd responded "Yes!" and Chrestman replied "Take it!"

(Photo courtesy of Department of Justice)

The trio later appears together with Kuehn motioning to Colon and grabbing a podium in order to block a police barrier from closing, according to federal authorities, and Colon then grabs a chair to block a separate barrier. 

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The FBI continues to search for other individuals involved in the Jan. 6 Capitol riot. 

Anyone with information on the whereabouts of individuals involved can submit a tip at https://tips.fbi.gov/. 

Fox News' Jake Gibson and Vandana Rambaran contributed to this report