The FBI has said it is pursuing a single suspect over the death of a police officer who was killed in the Capitol riots.
Footage shows the person spraying a substance at Brian Sicknick and several other officers during the chaos in Washington DC on 6 January, officials have said.
They added the suspect is also heard discussing the alleged attack before targeting the officers.
The person has not been identified and the FBI has not yet released the video to the public.
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A federal law enforcement source and another source close to the investigation confirmed the news on Saturday.
Mr Sicknick, 42, was injured while “physically engaging with protesters” before returning to his division’s office after the riot.
He then collapsed before being taken to a hospital where he died the next day.
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Investigators initially believed that Mr Sicknick was hit in the head with a fire extinguisher.
However, the idea that the police officer died after inhaling a chemical irritant has emerged in recent weeks as a new theory in the case.
The circumstances surrounding Mr Sicknick’s death remain unclear and a final cause of death has not yet been determined.
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Mr Sicknick was among five people who died when supporters of then-president Donald Trump stormed the Capitol Building to stop Congress from formally accepting the Electoral College votes that delivered last November’s election to Joe Biden.
Mr Trump had urged supporters on the National Mall to “fight like hell” to overturn his defeat.
Mr Sicknick, who served in the New Jersey Air National Guard before joining the Capitol police in 2008, was honoured by politicians this month.
He also had the rare distinction of lying in honour in the Capitol building’s rotunda.
House Speaker Nancy Pelosi said in remarks during the ceremony: “Our promise to Brian’s family is that we will never forget his sacrifice.
“Each day, when members enter the Capitol, this temple of democracy, we will remember his sacrifice and that (of) others that day, who fought so hard to protect the Capitol and the Congress.”
Mr Trump was impeached on an article charging him with “incitement of insurrection” over the 6 January riot but he was acquitted by the Senate.