Major new details emerge about assassination of insurance CEO as police explain what happened before killing

Police have uncovered new details about the assassination of a health insurance CEO who was shot dead in New York City this week.

What we already know is that in the early hours of Wednesday (4 December), somebody shot and killed health insurance CEO Brian Thompson as he was leaving his hotel.

The assassin then escaped on foot down an alleyway before getting onto an e-bike and riding it into Central Park, where CCTV footage lost track of them.

The bullet casings of the shots the assassin fired had the messages 'deny', 'defend' and 'depose' written on them. People think the message is a connection to a book called Delay, Deny, Defend which critiques the way health insurance companies operate.

'Delay, deny, defend' is a phrase which critics of the American health insurance system use to describe the tactics of healthcare insurance companies to avoid paying out on claims.

Police think they know how and when the suspect entered New York City. (NYPD)

Other items, including a phone and water bottle, were also recovered from the scene of the shooting and are being investigated for DNA.

Police had previously released what images they could of an individual thought to be the shooter, and now, it sounds as though they have learned more about the individual they suspect of being the assassin.

ABC 7 reports that the NYPD believes the suspect arrived into New York City on a bus from Atlanta in November, though at which point he boarded the bus is not clear.

He reportedly checked into the HI New York City Hostel on 30 November using an ID that wasn't his.

When the investigation looked into the name of the suspect, it did not match images taken by CCTV, ABC says police sources told them.

Their stay at the hostel did allow for the clearest picture of the suspect, as they pulled down a mask covering their face to speak with a receptionist while checking in.

An image of the suspect without a mask on taken at a New York hostel could be a new piece of the puzzle. (NYPD)

Police have described the shooting as a 'brazen, targeted attack', as the gunman appeared to be 'lying in wait for several minutes' outside the hotel where Brian Thompson was staying.

After the UnitedHealthcare CEO left the hotel, the shooter walked up behind him and fired three shots, then made his escape into Central Park and onto unknown whereabouts.

Thompson's wife Paulette said that he had received some threats, though their local police authority in Minnesota said that hadn't been reported to them.

UnitedHealthcare is the largest provider of Medicare Advantage plans in the US, and made $281 billion (£219 billion) in revenue last year.