Nick Buckles said he can't talk about why G4S messed up the Olympics contract because there is an ongoing review of what happened which isn't finished yet.
But he says the initial requirement of 2,000 staff was increased to have 15,000 staff on standby. He said they realised they had problems in early-July.
He emphasises that 7,000 military were always expected to work alongside G4S but additional troops were drafted in to help them with the shortfall.
He is spending quite some time saying how complex the Olympics contract was - maybe so, but they signed up to it.
The Olympics fiasco could hurt G4S' role in the public sector in the UK, making them a less tempting choice for government or councils.
Security company G4S has admitted the failure to fulfil its Olympics contract cost it in the region of £50 million.