WASHINGTON -- Six weeks after the Oklahoma City bombing, federal authorities have unearthed little evidence that the plot involved anyone other than the two men already charged, law enforcement officials say.
Nevertheless, investigators have refused to shelve their theory of a larger conspiracy, driven by a belief that so enormous a crime could not have been carried out by just two people and tantalized by witnesses who report fleeting glimpses of another suspect, still identified only as John Doe No. 2.
Law-enforcement officials hoped that they might be near a breakthrough two weeks ago, when a friend of Timothy J. McVeigh, one of the men already charged, began negotiations with prosecutors over the possibility of a plea agreement.
But that friend, Michael Fortier, has not reached any deal with the government, and the future of his discussions with the prosecution is uncertain, officials say.
Mr. Fortier has told the authorities that he accompanied Mr. McVeigh from Arizona to Oklahoma City to inspect the federal building there a few days before it was blown up April 19.
He also said Mr. McVeigh had earlier told him of plans to bomb an unspecified federal building. But he has denied active involvement in the plot.
By the account of some news reports, Mr. Fortier has also said he knew that Terry L. Nichols, the second man charged in the bombing, was to have a role in mixing the chemicals used to build the bomb. But the authorities have not confirmed these accounts.
Some investigators have said they are skeptical of Mr. Fortier's statements and have pressed him to submit to a lie-detector test and supply more specific information that can be independently corroborated. It is not clear whether he has agreed or even whether his limited admissions of knowledge place him in legal jeopardy. So far, he has not been charged with a crime.
Most investigators are satisfied with the evidence they have compiled against Mr. McVeigh. But some acknowledge that the evidence against Mr. Nichols is shaky, particularly for a prosecution in which Attorney General Janet Reno and President Clinton have promised to seek the death penalty.
In the case of Mr. McVeigh, investigators have determined that the clothes he was wearing when he was arrested on unrelated charges little more than an hour after the explosion had traces of chemicals that were also present in the detonating cord used in the bomb.
Moreover, a truck rental clerk in Junction City, Kan., has identified Mr. McVeigh in a lineup as the man who on April 16 rented the Ryder truck in which the bomb went off. The evidence against Mr. Nichols appears damaging but less convincing. At his home, investigators found receipts for large purchases of ammonium nitrate, plastic barrels and blasting caps like those used in the bombing. But his lawyer, Michael Tigar, has said in legal papers that the government has failed to demonstrate that Mr. Nichols knew of the bombing in advance.
All in all, government investigators, who seemed to be making quick progress when Mr. McVeigh was taken into custody as a bombing suspect two days after the blast, appear to have lost momentum.
Law enforcement officials deny that the investigation is in trouble or even that it has been slowed. Some say the appearance that it has lost momentum is misleading and that it has simply entered a quieter, grand jury phase in which prosecutors offer evidence in closed proceedings that will lead to indictments expected later this summer.
Officials also note that the prosecution team in Oklahoma City has been awaiting Joseph H. Hartzler, a federal prosecutor from Illinois appointed as the government's lead lawyer in the case. His arrival in Oklahoma City this week will most likely accelerate the pace.