WASHINGTON — Officer Brian D. Sicknick of the U.S. Capitol Police had multiple strokes hours after sparring with a pro-Trump mob during the Jan. 6 riot and died of natural causes, Washington’s medical examiner said on Monday.
The determination is likely to complicate the Justice Department’s efforts to prosecute anyone in the death of Officer Sicknick, 42; two men have been charged with assaulting him by spraying an unknown chemical on him outside the Capitol.
But an autopsy found no evidence that Officer Sicknick had an allergic reaction to chemicals or any internal or external injuries, the medical examiner, Dr. Francisco J. Diaz, told The Washington Post, which first reported his finding.
Still, Dr. Diaz added of the riot, “All that transpired played a role in his condition.” His office said that it attributes death to natural causes when it can be ascribed to disease alone and that “if death is hastened by an injury, the manner of death is not considered natural.”
Officer Sicknick was one of five people left dead after the attack. One rioter, Ashli Babbitt, was shot to death by another Capitol Police officer. Two others died of complications from heart disease and one death was accidental, Dr. Diaz has ruled.
Officer Sicknick died from “acute brainstem and cerebellar infarcts due to basilar artery thrombosis,” Dr. Diaz ruled, meaning a serious stroke. The artery that supplies the brainstem — the master controller of the body’s functions like breathing and heart rate — was blocked by a clot, and the blood flow to the cerebellum at the back of the brain was blocked.
Strokes that involve the brainstem are devastating and usually fatal, said Dr. Lee Schwamm, a stroke expert at Harvard. They typically are caused by atherosclerosis in the artery that feeds the brainstem or by a big clot that originates in the heart and lodges in that artery.
Two men were charged last month with assaulting Officer Sicknick, but prosecutors have avoided linking the attack to his death. He was injured “as a result of being sprayed in the face” with an unidentified substance, according to court papers.
The Justice Department has accused the suspects, George Pierre Tanios, 39, of Morgantown, W.Va., and Julian Elie Khater, 32, of State College, Pa., of working together “to assault law enforcement officers with an unknown chemical substance by spraying officers directly in the face and eyes.”
Mr. Tanios and Mr. Khater were charged with conspiracy to injure an officer, assaulting an officer with a dangerous weapon, civil disorder, obstruction of an official proceeding and other crimes related to violent conduct on the grounds of the Capitol.
The determination by Dr. Diaz is likely to be used by supporters of former President Donald J. Trump and others who have tried to downplay the storming of the Capitol.
About 140 police officers were assaulted during the riot, including about 80 from the Capitol Police and about 60 from Washington’s Metropolitan Police Department, according to the Justice Department. Two other officers who tried to stop the siege later died by suicide.
More than 410 people have been arrested in at least 45 states.
In a statement released on Monday, the Capitol Police force said it accepted the medical examiner’s findings but added, “This does not change the fact Officer Sicknick died in the line of duty, courageously defending Congress and the Capitol.”
Officer Sicknick joined the department in July 2008 and most recently served in its first responder’s unit, according to the force. He was an Air National Guard veteran who served in Saudi Arabia and Kyrgyzstan.
The Capitol Police had previously said in a statement that Officer Sicknick died from injuries suffered “while physically engaging with protesters.”
The men sprayed Officer Sicknick and others minutes before the police line on the west side of the Capitol collapsed and rioters gained control over it, video obtained by The New York Times has shown.
Officer Sicknick and two other police officers were injured and temporarily blinded “as a result of being sprayed in the face” with an unidentified substance by Mr. Khater and Mr. Tanios, according to the F.B.I.
The men were seen on video early in the afternoon of Jan. 6 standing several feet from police officers, including Officer Sicknick, the F.B.I. has said. In a video of the attack, Mr. Khater said “give me that” and reached into Mr. Tanios’s backpack, the F.B.I. said. Mr. Tanios protested that it was too early, apparently to attack the officers with the spray. Mr. Khater countered that he had just been sprayed and held up a can of chemical spray.
The authorities have said that Officer Sicknick and two other officers, who were all standing near Mr. Khater, reacted to being sprayed in the face and were forced to back away.
After collapsing that night, Officer Sicknick was rushed to the hospital, where he later died. Investigators opened a homicide investigation immediately after his death with the hopes of determining if anyone who fought with him contributed to his death.
Officers have described the Jan. 6 events as “medieval” because of hand-to-hand combat and the use of blunt objects including baseball bats, flag poles and pipes as weapons.
The spray is extremely unlikely to have caused Officer Sicknick’s strokes, Dr. Schwamm said. The most likely explanation, Dr. Schwamm said, was a terrible coincidence of timing when blood clots that occurred for other reasons arose when he was in a terrifying situation.
If Officer Sicknick’s strokes had been caused by the attacks, the most likely explanation would be a tear in the artery that feeds the base of the brain. Clots can form when that happens and travel into a large artery that feeds the brain and block it.
Such cases are unusual, Dr. Schwamm said, but can occur when people violently twist and turn. A dissection would have been seen on an autopsy or imaging if the medical examiner looked at the entire vertebral artery, from its bottom at the back of the neck to where it enters the brain at the base of the skull.
Gina Kolata contributed reporting.