Five protesters made it to the chamber
Wednesday's raid on the Commons chamber by pro-hunt demonstrators was a carefully-planned operation.
But the protesters also claimed later that they found it surprisingly easy to gain entrance to what should be one of the most secure areas in the country.
The raid began when eight protesters dressed as surveyors, in business suits, hard hats and fluorescent bibs, arrived at a public entrance to the Commons.
The protesters will have faced airport-style security screening but once inside, were, apparently, able to roam at will.
They were reportedly posing as building contractors invited to a site meeting in the Committee corridor, where building work is taking place.
According to press reports, they were carrying a forged letter purporting to be from two MPs, Tory Sir Peter Tapsell and Labour's Kerry Pollard, addressed to a "Mr C Ferie" and headed "Re - Site Meeting All-Party Electrical and Skills Group".
According to the Daily Telegraph, the letter said: "We are trying to show the contractors the re-wiring and construction work throughout the building."
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It said the contractors would be greeted in the central lobby, between the Lords and Commons, and given a tour of the building, but added: "Failing that, please head directly to committee room 12."
The protesters passed through St Stephens Hall and the central lobby apparently unchallenged.
Then, according to Commons Speaker Michael Martin, they were "led into the small stairway to the north end of the corridor - probably by a passholder who was clearly exceeding his or her authority".
It is not clear whether the passholder who apparently helped the intruders was an MP, a reporter or an employee of a member, he said.
The protesters are thought to have discarded their suit jackets and hard hats in a committee room, although other reports suggest they did this later, as they were about to enter the chamber.
The eight then passed through a security door that blocks off access to the Ladies' Gate stairway, that leads directly into the Commons chamber itself.
According to some press reports the door, which is security pass-protected, was faulty and just needed a "hard shove" to push it open.
A protester in white T-shirt points a finger at MPs in the Commons
But BBC reporters who tested it in the immediate aftermath of the raid said it was working correctly, suggesting that it had either been left open or that someone had used their pass to let the protesters in.
Once through the door and down the staircase, the protesters passed down the corridor immediately behind the speaker's chair.
At this point, according to eyewitnesses, they were confronted by Commons gatekeepers who managed to stop three of them.
But five of them got through. Four ran out into the chamber from the "No" lobby behind the speaker's chair, while another wrestled past a doorkeeper from a different entrance.
The original plan, according to press reports, had been to get as many as 10 protesters into the chamber and barricade themselves in.
It later emerged the BBC had been given a tip-off about the planned intrusion on Wednesday morning by one of the protesters and told of a "dry run" carried out on Tuesday.
Otis Ferry, the 21-year-old son of rock star Bryan Ferry, had told a BBC reporter the group planned to dress as builders and gain access using a forged letter from an MP.
St Albans MP Kerry Pollard, one of the two members purported to have sent the invitation told the Telegraph it was a forgery.
"I don't even think there is such a thing as the all-party electrical and skills group. It's an outrage," he added.
He said he had no idea how his name and forged signature had come to be on the letter, except that he was the secretary of the all-party construction skills group, which does exist.
PROTESTERS' APPARENT ROUTE
1: Eight protesters come through public entrance, dressed as builders, carrying a forged letter inviting them to a meeting
2: They go upstairs to Committee Corridor where building work is taking place
3: The protesters somehow pass through a door which has a swipe card lock and head downstairs
4: Four protesters enter chamber from 'No' voting lobby, three are detained outside the door
5: Another one comes in via main chamber entrance