Authorities are piecing together what they know about the man shot dead outside the US Capitol building after he launched a fatal attack on police officers.

US Capitol Police officer William Evans died in hospital after a man rammed a car into two officers and “lunged” towards them with a knife in his hand at around 1pm local time on Friday. He was then shot dead by one of the officers.

Four senior law enforcement officials briefed on the investigation have named the suspect as Noah Green, a 25-year-old from Indiana, according to NBC News.

So far, there are few details about who he was but an investigation has been launched to work out the motive behind the attack.

Green is thought to be originally from Indiana.

His Facebook page shows him wearing American football kit and he was said to be unemployed and dealing with a number of issues.

He was not known to either local police or the US Capitol Police and the attack is not thought to be terror-related, authorities have said.

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Sky correspondent Alex Rossi said investigators will now be “trying to build up a picture of this person”, using information including his social media and phone records.

Authorities will be trying to work out details including who his friends and family were, and who he spoke to in the days leading up to the attack, Rossi added.

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‘One of our officers has succumbed to his injuries’

In a news conference Capitol Police acting chief Yogananda Pittman said the suspect had rammed his car into two officers before hitting a barrier.

He then got out of the car and started to “run aggressively” at officers with a knife in his hand, “lunging” towards them.

Washington DC police chief Robert Conti said: “Clearly this was someone who was actively trying to get to whoever or whatever.”

Former CIA covert operations officer Michael Baker said there would be “a lot of speculation” as to the motive for the attack, and he expected there would be a “jump” to the suggestion the suspect was a far-right supporter.

However, he said the timing of the attack could potentially “point to a mental health issue”.

The House and Senate are currently on recess so the politicians are not currently in the building.

“It’s a quiet time up there and you would assume if it was a more nefarious plot they would have understood that and chosen a time it was more busy,” Mr Baker told Sky News.