The FBI man in charge of collecting evidence from the government building destroyed by the Oklahoma bomb has called for the case to be reopened.
Former deputy assistant director Danny Coulson has told the BBC programme The Conspiracy Files that he questions whether everyone involved was caught.
The attack in 1995 killed 168 people, including 19 children.
Gulf war veteran Timothy McVeigh was convicted of the attack and sentenced to death by lethal injection.
Mr Coulson said a federal grand jury is now needed to find out what really happened: "We have victims here and we have victims' families and we don't even know the answers. And the answer is frankly for a federal grand jury."
He argues this is the only way to prove whether other people were involved in the bombing in a wider conspiracy beyond Timothy McVeigh and Terry Nichols, who was also convicted of manslaughter and conspiracy and sentenced to life in jail.
Mr Coulson spent 31 years in the FBI. Between 1991 and 1997 he was the deputy assistant director of the Criminal Division of the FBI in Washington, responsible for all violent crime cases in the United States.
Mr Coulson said there were some "very strong indicators" that other people were involved with Timothy McVeigh.
The FBI interviewed 24 people who claimed to have seen McVeigh in Oklahoma City with someone else on the morning of the attack, yet the only known accomplice of McVeigh, Terry Nichols, was at home in Kansas over 200 miles away on that day.
The FBI's investigation concluded that the eyewitnesses were unreliable. However, Danny Coulson says they were "extremely credible" and had no reason to make it up.
"If only one person had seen it, or two of three, but 24? Twenty four people say yes I saw him with someone else. That's pretty powerful," he said.
Mr Coulson also says that FBI headquarters closed down part of their own investigation into a white separatist community called Elohim City, which conspiracy theorists believe was involved in the attack, with government knowledge.
Mr Coulson said: "It wasn't until after I had left the investigation, some months later, that I know FBI headquarters told them to close down the investigation in Elohim City which has some very significant connections to Mr McVeigh and previous bombing attempts.
"Never in my career did I have FBI headquarters tell me not to investigate something."
Jon Hersley, the FBI's leading case agent in the Oklahoma City bombing investigation disputes this.
"There was none of that whatsoever. In fact, in this investigation I feel like we uncovered more rocks than have ever been uncovered in the history of the FBI and at times we over-investigated parts of the case."
Last December a US Congressional report found no conclusive evidence of a wider conspiracy, but the report concluded that "questions remain unanswered and mysteries remain unsolved."
The report by the Oversight and Investigations Subcommittee of the House International Relations Committee said the FBI was "not justified in calling off any further investigation" into McVeigh's possible accomplice, and concluded that more investigation was needed.
The Conspiracy Files: Oklahoma Bomb was broadcast on Sunday, 4 March 2007 at 2100 GMT on BBC Two.