By Peter Biles
BBC world affairs correspondent
Jack Straw's evidence was the most crucial yet
Crowd control barriers were being delivered to the QEII Centre in Westminster at 7am on Monday morning.
It was a clear sign that we are really getting down to business at the Iraq inquiry.
When Tony Blair's former chief-of-staff, Jonathan Powell, arrived on foot at lunchtime, he found his route blocked by the mini-fortress around the main entrance.
His hope of quickly slipping into the building with anonymity was scuppered as he backtracked down the footpath, with press photographers in pursuit.
Mr Powell was a key figure in Downing Street during the Blair years. Just how important was revealed half an hour into his evidence session when he was questioned about the now infamous weekend at President Bush's ranch in Crawford, Texas, in April 2002.
We first heard about this private Bush-Blair encounter from Sir Christopher Meyer, the former British ambassador in Washington, who gave evidence in November.
Jonathan Powell however, had a different recollection of who was closest to the two leaders at the time.
"I was at Crawford, David Manning (Tony Blair's foreign policy adviser) was at Crawford, Christopher Meyer was not at Crawford. He was at Waco, about 30 miles away."
So at times, the inquiry is all about who said what to whom, as well as when and where.
We discovered this week that Geoff Hoon, the former Defence Secretary, was in the Ukrainian capital, Kiev, when the government's controversial dossier, containing the 45 minutes WMD claim, created a major splash in the British press in September 2002.
In Kiev, Mr Hoon said he never saw the newspapers and missed the story entirely.
Geoff Hoon, who describes himself as "a lapsed lawyer", recalled being interviewed on television by Jonathan Dimbleby in 2002.
He said he was being "pushed quite hard" on the legality of using force against Iraq.
"I was trying quite hard not to answer any questions, and that's quite difficult when there are only two of you having a conversation".
Mr Hoon was more forthcoming over the six hours he was in front of the inquiry. But he still managed to give an impression of a man who was perhaps content not to have been involved in key decisions or present at crucial meetings.
As a witness, Mr Hoon was never going to make the headlines this week. But he did fire off a couple of broadsides towards the end of his marathon session.
There was implicit criticism of Gordon Brown for not adequately funding the Ministry of Defence when Mr Brown held the post of Chancellor of the Exchequer.
And there was clear disquiet from Mr Hoon when Tony Blair announced in 2004 that there would be a significant troop commitment to Afghanistan before the draw-down of troops in Iraq.
Jack Straw's appearance on Thursday was the most important evidence session so far.
A 25 page memorandum released beforehand, set the tone for the afternoon.
Geoff Hoon was in Kiev during the furore over the intelligence dossier
The Iraq war had been "the most difficult decision I have ever faced", and he deeply regretted "the grave loss of life".
The former foreign secretary was determined to demonstrate that he had never been a "regime changer" on Iraq. That would have been "improper and unlawful", said Mr Straw.
Sir John Chilcot and his colleagues wanted to know if Tony Blair had held a similar view.
"I think the best way to find that out is to ask him", answered Mr Straw.
"We obviously will", said his inquisitor, Sir Roderic Lyne.
At times, Jack Straw took his time to answer the questions, and like any trained lawyer, chose his words carefully. But the inquiry came close to exposing differences between Mr Straw and Mr Blair.
Mr Straw admitted that had he been writing notes to President Bush in 2002, he might not have worded them in the same way as Tony Blair.
The bottom line from Mr Straw: "Look, we are two different people".
The Iraq war appears to have weighed heavily on Jack Straw.
He said he never thought it was a resignation issue, but he painted a picture of someone drawn reluctantly into a conflict. He would have much preferred to continue working along the diplomatic track.
This past week's evidence - dominated by Jonathan Powell, Geoff Hoon and Jack Straw - has laid the ground for Tony Blair's appearance on January 29.
That, in turn, puts the focus on Gordon Brown, who, it was confirmed on Friday, is set to face inquiry several months earlier than anyone anticipated.