Investigators now believe the bombing was self-sponsored and financed by arms deals made on a meandering circuit of gun shows across the US involving the two accused suspects, Timothy McVeigh and Terry Nichols, and Michael Fortier, an army friend of Mr McVeigh who has not yet been charged.
"At this juncture, determining how the plotters came up with the money is vital," said one source close to the investigation.
It is estimated that the raw materials for the fuel and fertiliser bomb would have cost no more than $3,000 (pounds 1,920). Even with travel expenses and motel accommodation, the project could have cost less than $10,000.
"Every stop seems to lead us to a new stop and more questions," said the source. "Guns were their avocation. It seems it was their livelihood too.''
As far back as 1993 Mr McVeigh, using the alias Tuttle - from the revolutionary character played by Robert de Niro in the film Brazil - advertised weapons, including an anti-tank gun, for sale in Spotlight, an ultra-conservative publication.
The post office box that Mr McVeigh used was in Kingman, Arizona, where much of the investigation is taking place. The FBI is concentrating on establishing a time line for the alleged conspiracy.
Mr McVeigh is believed to have been involved in the robbery of an Arkansas gun collector last November. The key to the collector's safe was found in a barrel at Mr Nichols's farm in Michigan, and later one of the stolen guns was allegedly sold by James Rosencrans, another McVeigh friend, who lives in Kingman.
In December, Mr Fortier told friends he was going on holiday to Florida. Instead, he turned up in Kansas, not far from Mr Nichols's home, where he rented a car that has since been found to contain traces of gun oil. By March, he was also on the gun show circuit, selling weapons.
The search for clues continues in the desert near Kingman, where it is thought that a duffel bag, possibly containing bomb fuses and weapons, was buried by Mr McVeigh before the bombing. The search has been hampered by snakes, which infest the area.
A local television station reported on Thursday that FBI investigators had found a pile of ammonium nitrate, one of the constituents of the bomb, in the desert. Station KNXV in Phoenix reported that Mr Rosencrans had dumped the chemical three days after the bombing, on the instructions of Mr Fortier, who is trying to negotiate a plea bargain with prosecutors in return for his co-operation.
On Friday, FBI agents sifted through homeless-shelter records on Francis "Rocky" McPeak, whose house was damaged in February by a bomb made of ammonium nitrate and diesel fuel.
The federal grand jury has until 11 August to decide whether to charge others beyond Mr McVeigh and Mr Nichols. Mr McVeigh's lawyers are fighting a request for a sample of his handwriting, which the FBI believes will match that on the rental contract for the Ryder truck that contained the bomb.
Federal prosecutors, meanwhile, are considering whether to seek the death penalty against Mr McVeigh and Mr Nichols for their alleged part in the bombing - something the Attorney-General, Janet Reno, asked for shortly after the explosion.