The official version, the conspiracy theories and the evidence surrounding the Oklahoma bombing.
Did Tim McVeigh have an accomplice on the day?
At least 24 eyewitnesses say they saw Tim McVeigh with one or more accomplices in Oklahoma City on the morning of the attack. But no one was ever identified.
It was not Terry Nichols, the known accomplice of Tim McVeigh. He was at home in Kansas over 200 miles away on that day.
The FBI called the mystery person or persons "John Doe II". If they existed it would mean the FBI and the government were wrong to have pinned responsibility for the attack on just Tim McVeigh and Terry Nichols, and it would suggest a wider plot involving people who have never been brought to justice.
The FBI eventually concluded that the testimonies of these eyewitnesses conflicted with each other.
One of them, Mike Moroz, claimed Tim McVeigh stopped to ask directions to his target, the Alfred P Murrah Building. But the FBI said McVeigh was already familiar with this government building - he had pointed it out to Terry Nichols - and he would not have needed to stop and ask the way.
Jon Hersley, one of the FBI's main agents in the investigation, says these eyewitnesses were trying to help and some stretched their memories to do this.
Many put McVeigh in different places at the same time on the morning of the 19 April, 1995.
The FBI says it considered every one of these sightings, and if any had been reliable they would have used them in the trials.
However, Danny Coulson, a former assistant deputy director of the FBI, says many of these witnesses were credible.
If only one or two had seen McVeigh with another person they could have got it wrong. But can all 24 be mistaken?
Intriguingly, even the Oklahoma County Grand Jury, which looked into conspiracy theories surrounding the bombing and concluded there were none, said in its final report: "We cannot finally put closure to the question of the existence of John Doe II and are confident that if any new evidence comes to light, they [the FBI] and other law enforcement agencies will pursue these leads."
Was the government warned about the attack?
An informant for the Bureau of Alcohol, Tobacco and Firearms (BATF), Carol Howe, provided information to the authorities before the attack that the federal building in Oklahoma was a potential target for the extreme right.
The FBI denied this. They say Carol Howe was "non specific" about potential attacks, she never mentioned the federal building in Oklahoma City, nor did she identify Tim McVeigh - prior to the bombing.
The FBI would also later question Ms Howe's reliability and claim that after the bombing she changed her story to fit the facts.
For many, whether or not you believe in a bombing conspiracy seems to rest on whether you believe Carol Howe's version of events.
Those she points the finger at as being part of the conspiracy, such as Andreas Strassmeir and the inhabitants of a white separatist community, called Elohim City say her allegations were false.
But for conspiracists, Carol Howe is the Holy Grail. They say months before the bombing she was informing the BATF that Elohim City was preparing for "war" and that Robert Millar - the spiritual leader of the settlement - had "brought forth his soldiers and instructed them to take whatever action is necessary against the US government."
She also identified Andreas Karl Strassmeir as Elohim City's Head of Security and claimed he wanted to "forcibly act to destroy the US government" with direct actions "such as assassinations, bombings, and mass shootings."
Strassmeir denies all of this. He told The Conspiracy Files that at the time many people were engaged in anti-government talk and much of this anger was directed towards the BATF, the FBI and the federal government because of the botched raid at Waco in 1993.
But, says Strassmeir, there is a big difference between saying something and actually planning or carrying it out.
The FBI says it investigated Elohim City and while Robert Millar had allowed criminal elements to stay there from time to time, there was no credible evidence that the residents themselves had engaged in violent acts against the government.
Did a German secret agent provoke the attack?
Andreas Strassmeir, also known as "Andy the German", is key to this conspiracy theory.
It is claimed he was a German secret agent brought to America in the early 90s to keep watch on the American far right.
Working jointly for the FBI and the German authorities his role was to report to them about the activities of American Neo-Nazis.
According to the conspiracy theory Strassmeir was an "agent provocateur", not only observing the activities of the far right but also encouraging them to do something illegal.
It is suggested that after the disaster at Waco, federal law enforcement agencies needed to rescue their reputation with a high profile success. What better than to foil a bomb plot in Oklahoma City.
But the authorities failed to stop McVeigh and his accomplices in time, 168 people died and according to the conspiracists the government has been covering up ever since.
The mystery is further fuelled by the fact that Andreas Strassmeir was not questioned by the FBI immediately after the attack.
Yet Strassmeir met Tim McVeigh at a gun show in 1993, Tim McVeigh had tried to call him on 5 April 1995 and the undercover informant Carol Howe said Strassmeir had plans to "destroy the US government" with direct actions and operations "such as assassinations, bombings, and mass shootings."
Strassmeir was eventually questioned 9 months after he had left the US and only over the phone.
Conspiracy theorists say there can only be one reason for this, namely that he is protected because he is an agent.
Strassmeir denies all of this and says he was not involved with McVeigh or the bombing. It was a chance meeting at a gun show and he never saw or spoke to him again.
He says the conspiracists only give him two options, either he is a government agent or he is a terrorist.
Strassmeir says there is a third option, that he is an innocent man. He told The Conspiracy Files that if he was a terrorist involved in the bombing he would have been handed over to the Americans long ago and that if he was an agent he would have been "taken out" or living under an assumed identity in a foreign land.
The FBI categorically denies the bombing was a sting that went wrong. Jon Hersley, the lead case investigator in the Oklahoma City Bombing Investigation says the FBI does not play games with people's lives and would not get involved in anything of that nature.
Furthermore, the FBI says that if it had possessed any information on a potential bombing, immediate action would have been taken against those individuals to prohibit anything happening.
Was Elohim City home to conspirators?
Elohim City is a white separatist, Christian Identity settlement in the Ozark Mountains in Oklahoma, on the border with Arkansas.
After the bombing the community received substantial public attention when it was revealed that Tim McVeigh had called Elohim City.
He telephoned the Millar residence at Elohim City on 5 April, 1995, two weeks before he attacked the Alfred P Murrah and shortly after McVeigh had called a Ryder Truck Rental office.
Elohim City's then spiritual leader the Rev Robert Millar said he had no idea whom McVeigh was calling for or why. He also said they had no connection to him.
It later transpired that McVeigh was calling for Andreas Strassmeir, who was "head of security" at Elohim City.
Strassmeir had met McVeigh at a gun show in Tulsa, Oklahoma in 1993 and had handed him a business card for the community.
It was also revealed that the Bureau of Alcohol, Tobacco and Firearms had had an undercover informant in Elohim City called Carol Howe who told her handlers in 1994 that Elohim City was preparing for war and Strassmeir wanted to destroy the "US government" with direct actions "such as assassinations, bombings, and mass shootings"
Furthermore, the day after McVeigh blew up the government building, a man was buried at Elohim City who had plotted to attack that same building back in 1983 - he was called Richard Wayne Snell and Elohim City's spiritual leader Robert Millar had acted as his spiritual advisor.
There is further strange coincidence involving Elohim City. Several members of a group known as the Mid West Bank Robbers or the Aryan Republican Army regularly stayed there and one of them even shared a room with Andreas Strassmeir.
The Aryan Republican Army was formed in 1993 and their primary objective was to rob banks to fund attacks on federal buildings. Peter Langan, their leader would later claim that they had links with Timothy McVeigh.
But the FBI's two main case investigators in the bombing investigation told The Conspiracy Files that Elohim City was an interesting and significant place with a lot of characters either living there or just passing through.
But they argue that does not necessarily mean they acted in concert or in conjunction with each other, and the evidence suggests otherwise.
Was there a link to an earlier bomb plot?
Richard Wayne Snell was a member of the heavily armed, white supremacist, Christian Identity, paramilitary survivalist group known as the Covenant, the Sword and the Arm of the Lord or CSA.
The group was founded in 1970 and consisted of between 90 and 120 men, women and children.
They were located in Oklahoma near the Arkansas border on a secluded 220 acre farm, called Zarephath-Horeb.
They stockpiled food, weapons and trained themselves in military and survival procedures.
The CSA believed in a doomsday scenario, where society would suffer economic collapse or nuclear war.
In the chaos the panicked masses would rove the country looting for food and protection.
But the CSA also wanted to start a war with the federal authorities. The target they chose for the opening attack was the same building McVeigh would later blow up, the Alfred P Murrah Building in Oklahoma.
The attack was to have been carried out by Richard Wayne Snell in 1983. Snell thoroughly checked out the building.
However, the attack was never carried out after a malfunctioning rocket launcher to be used in the attack backfired during training.
Richard Wayne Snell was later arrested and sentenced to death for the murder of a black highway state trooper.
Snell was executed on 19 April 1995 hours after Tim McVeigh had attacked the Alfred P Murrah Building.
The conspiracy theory suggests McVeigh targeted this building as a "going away present" for Snell.
While on death row Snell's spiritual advisor was Robert Millar of Elohim City - Elohim City had developed close ties with the CSA in the 1980's. Millar acted as a "messenger" between Tim McVeigh and Richard Wayne Snell.
Snell's prison log shows that on 19 April 1995, around noon he asked for the television to be put on, and as he watched the news of the Oklahoma bombing Snell was seen to be "smiling and chuckling."
But the FBI says there is no evidence to suggest that McVeigh had ever made contact with Snell.
McVeigh chose the Alfred P Murrah building as a target because that was where he thought the order for the raid on Waco which ended up with the deaths of 80 people including 21 children had come from.
Were the Midwest Bank Robbers involved?
The Mid West Bank Robbers who were also known as the Aryan Republican Army (ARA), were a white supremacist group that robbed banks across middle American in the mid 90s to fund attacks on federal buildings.
The group's leader was called Peter Langan aka Commander Pedro, who together with Richard Lee Guthrie, Scott Stedeford, Kevin McCarthy and Michael Brescia, robbed around 19 banks between 1993 and 1996 netting $250,000 (£127,000).
Possible connections between the Midwest Bank Robbers and the Oklahoma City bombing came to light after Peter Langan claimed there was a connection between his group and Timothy McVeigh.
Conspiracy theorists argue that by observing the movements of the Mid West Bank robbers between 1993 and 1995 and those of Tim McVeigh over the same period it is possible to see that they were all in the same place at the same time on a number of occasions.
These places are not big cities, but smaller places such as Fayetville, Arkansas or Kingman, Arizona.
It is argued that McVeigh and the Aryan Republican Army were all members of the same movement of anti-government activists which numbered around 5,000 - 10,000.
McVeigh and the ARA were some of the most violent of these activists and, it is claimed, the chance that they would be in the same geographical location on the same day is highly improbable unless they were there for a reason.
The FBI says that being in the same place at the same time does not mean these people met. They say this evidence is thin and circumstantial.
The FBI found no phone calls between McVeigh and Langan or any of his associates.
A search through millions of motel records failed to show any link between McVeigh and Langan's group.
The FBI concluded that the reason there are no links, no common calls is because they did not happen and they simply never met.
However, conspiracists theorists also point to an FBI interview with Timothy McVeigh's sister in which she claimed her brother had said he was involved with some bank robbers and had given her some money from the proceeds to launder.
The Conspiracy Files: Oklahoma Bomb was broadcast on Sunday, 4 March 2007 at 2100 GMT on BBC Two.