Capitol Police officer Brian D. Sicknick suffered two strokes and died of natural causes a day after he confronted rioters at the Jan. 6 insurrection, the District’s chief medical examiner has ruled.

The ruling, released Monday, will make it difficult for prosecutors to pursue homicide charges in the officer’s death. Two men are accused of assaulting Sicknick by spraying a powerful chemical irritant at him during the siege, but prosecutors have not tied that exposure to Sicknick’s death.

In an interview with The Washington Post, Francisco J. Diaz, the medical examiner, said the autopsy found no evidence the 42-year-old officer suffered an allergic reaction to chemical irritants, which Diaz said would have caused Sicknick’s throat to quickly seize. Diaz also said there was no evidence of internal or external injuries.

Members of Congress paid their respects on Feb. 3 to Brian D. Sicknick, the Capitol Police officer who suffered fatal injuries Jan. 6. (The Washington Post)

Diaz said Sicknick suffered two strokes at the base of the brain stem caused by a clot in an artery that supplies blood to that area of the body. Diaz said he could not comment on whether Sicknick had a preexisting medical condition, citing privacy laws.

In the days after the riot, police and a Justice Department official attributed Sicknick’s death to his efforts to contain the riot.

Democratic House managers arguing for then-President Donald Trump’s impeachment said Sicknick was killed by rioters, citing a New York Times story that said police initially believed Sicknick had been struck with a fire extinguisher. The Times later updated the story saying there was no evidence of blunt-force trauma.

But exactly what caused Sicknick’s death has remained unclear for more than 14 weeks.

Efforts to reach Sicknick’s family on Monday were not successful, and Capitol Police said in a statement that the family had asked for privacy.

Diaz’s ruling does not mean Sicknick was not assaulted or that the violent events at the Capitol did not contribute to his death. The medical examiner noted Sicknick was among the officers who engaged the mob and said “all that transpired played a role in his condition.”

The Capitol Police said in its statement that the ruling “does not change the fact Officer Brian Sicknick died in the line of duty, courageously defending Congress and the Capitol.” The agency said it will “never forget Officer Sicknick’s bravery, nor the bravery of any officer on January 6, who risked their lives to defend our democracy.”

Christopher Macchiaroli, a former federal prosecutor who handled violent crime cases before grand juries in D.C. Superior Court and U.S. District Court, said a ruling of a death by natural causes “does make it more difficult to bring a homicide prosecution.”

Macchiaroli said additional evidence of some conduct by rioters could emerge independently, which prosecutors could argue contributed to the strokes. But he said that “any defense attorney . . . would use the medical examiner’s conclusions as clear-cut evidence of reasonable doubt.”

In explaining the decision, the medical examiner’s office provided an updated timeline leading up to Sicknick’s death. A statement says Sicknick collapsed 7 hours and 40 minutes after he was sprayed, and then died nearly 24 hours after that.

Sicknick was among hundreds of officers who confronted the violent mob that took over the Capitol, seeking to overturn the election Donald Trump had lost. Nearly 140 officers were assaulted, authorities said, facing some rioters armed with ax handles, bats, metal batons, wooden poles, hockey sticks and other weapons.

In early February, Sicknick, who grew up in South River, N.J., was honored at the U.S. Capitol. His remains were interred at Arlington National Cemetery.

His death is being investigated by D.C. police, who handle all deaths in the District; the Capitol Police; and the FBI.

In a Jan. 8 statement, then-acting U.S. attorney general Jeffrey A. Rosen said Sicknick died of “the injuries he suffered defending the U.S. Capitol.” He promised that local and federal authorities would “spare no resources in investigating and holding accountable those responsible.”

The Capitol Police said in a statement soon after the riot that Sicknick “was injured while physically engaging with protesters.”

Christopher Geldart, the District’s acting deputy mayor for public safety, said Monday that the medical examiner’s office “took the appropriate amount of time to evaluate all the evidence” in Sicknick’s death, which he said including reviewing videos, statements from officers and the results of toxicology screens.

Geldart said Diaz “felt he was able to make this call in good conscience.”

In March, two men — Julian Elie Khater, 32, of Pennsylvania, and George Pierre Tanios, 39, of West Virginia — were accused of assaulting Sicknick with chemical spray during the riot.

At the time of the arrests, the autopsy had not been completed. Tanios’s attorney, Beth Gross with the federal public defender service, declined to comment on the ruling. Attorneys for Khater on Monday did not respond to inquiries.

Arrest papers allege that Khater and Tanios were at the Lower West Terrace of the Capitol at 2:14 p.m. on Jan. 6, where Sicknick and other officers were standing behind metal bicycle racks.

“Give me that bear s---,” Khater said to Tanios on a recorded video, according to court documents.

About nine minutes later, those documents say, after Khater said he had been sprayed with something, Khater is seen on video discharging a canister of a toxic substance into the face of Sicknick and two other officers.

In a statement after Sicknick died, his family said, “Brian is a hero and that is what we would like people to remember.” The family said at that time that the “direct causes” of his injuries were unknown.

Sicknick grew up in a New Jersey borough south of New Brunswick, the youngest of three brothers. He is survived his siblings, Ken and Craig, his parents Charles and Gladys Sicknick, and his companion of 11 years, Sandra Garza.

Sen. Cory Booker (D-N.J.), paid tribute to Sicknick on the Senate floor on Jan. 19, saying the officer understood “that wearing that uniform, wearing that badge, that you had a sacred duty to protect this sacred space.”

The senator described Sicknick’s death as a “crime” that “demands the full attention of federal law enforcement.” He said that “when white supremacists attacked our nation’s capital, they took the life of one of our officers. They spilled his blood, they took our son away from his parents. They took a sibling away from their brothers.”

Sicknick joined the New Jersey Air National Guard in 1997 and had been assigned to the 108th Air Refueling Wing at Joint Base McGuire-Dix-Lakehurst in New Jersey. The Guard said he deployed to Saudi Arabia in 1999 and to Kyrgyzstan in 2003.

Although Sicknick supported Trump, those who encountered him said his political views did not align neatly with one political party. Messages he sent to his congressman, Rep. Don Beyer (D-Va.), were “polite and measured,” according to a spokesman for the lawmaker. Sicknick opposed impeachment for Trump and favored gun control. He was concerned about animal cruelty and the national debt.

During the ceremony in February honoring Sicknick at the Capitol, fellow officers, lawmakers, and President Biden and first lady Jill Biden paid their respects. In February, Senate Majority Leader Charles E. Schumer (D-N.Y.) described Sicknick as a “peacekeeper, not only in duty but in spirit.”