In July 2002, eight months before the war to oust Saddam Hussein, a convoy of Land Cruisers, Jeeps and a truck made a 10-hour journey over the border from Turkey into northern Iraq. They carried a group of CIA operatives.

One of them, who goes by the nickname of Tim, had been designated CIA base chief for Sulaymaniyah, the Kurdish town half way to Baghdad. A former navy Seal, he was in his thirties and spoke fluent Arabic.

Initially the work was difficult, but when Tim put it out that he would pay for information — the CIA gave him a war chest of $32m in $100 bills — things looked up.

By early 2003 Tim had a network of nearly 90 agents of such value that the CIA called them the Rockstars. They included senior Iraqi military officers and members of Saddam’s personal protection service, the Special Security Organisation (SSO). Tim equipped them with Thuraya satellite phones.

On Tuesday March 18, 2003, a call came through to Tim’s radio: “Pistachio, this is Jonestown!” A Rockstar agent, an SSO officer who was running part of the communications links that Saddam used as he moved between palaces and other locations, had important news.

He said that Saddam was at Dora Farm, a complex southeast of Baghdad on the bank of the Tigris River that was used by his wife. Dora Farm had the SSO codeword Umidza, meaning “slaughterhouse”.

The Rockstar’s information came from a sub-source called Rokan who was in charge of security at the “slaughterhouse”. Rokan had a Thuraya phone that could be geo-located on a CIA video display; it confirmed he was at the farm.

Tim passed the report on to Saul, the senior CIA officer running the Iraq operation at the agency’s headquarters in Langley, Virginia. It went straight to George Tenet, the CIA director, who tipped off the White House.

The information arrived at a tense moment in the final countdown to war. The previous evening, March 17, President Bush had gone on television to give Saddam and his two sons, Qusay and Uday, 48 hours to leave power.

By the morning of Wednesday March 19 there was more news from Dora Farm. Another Rockstar who had gone down to help with communications had noticed a significant security detail. They were stocking food and supplies. It looked like a family gathering.

Saul checked the latest overhead imagery of Baghdad. Lo and behold, under the palm trees at Dora Farm were 36 security vehicles!

The countdown to war was ticking. At least 31 teams of Special Operations Forces entered Iraq on schedule in the west and north as Tim received his next Rockstar report. It said Rokan had seen Saddam leave the slaughterhouse to attend meetings but would be back to sleep there with Qusay and Uday.

When this reached Washington, it galvanised Bush’s team. Within an hour Tenet, Saul, John McLaughlin, the deputy CIA director, and Donald Rumsfeld, the defence secretary, were briefing the president in his dining room.