LAW ENFORCEMENT : Bomb Probe: Grim Search for a Breakthrough : A month later, officials admit they’re still baffled about identity of ‘John Doe No. 2.’ Colbern’s detailed diary remains a key focus.

A month after a truck full of fertilizer and diesel fuel turned the Alfred P. Murrah Federal Building into a gravestone, the investigation of the Oklahoma City bombing has become a grim, grinding search for a breakthrough.

With two prime suspects in custody and a small army of federal agents still in the field pursuing thousands of tips and leads, officials are far from facing a dead-end.

Still, despite weeks of publicity and the most sweeping manhunt in American history, authorities acknowledge they know no more than they did three weeks ago about the baffling figure called “John Doe No. 2.”

They also remain uncertain exactly how many people may have been involved in the plot. Nor are they sure whether some of those under scrutiny played central roles or instead had only peripheral parts or some knowledge of the crime beforehand.

Indeed, sources close to the investigation say they fear that one of those now in custody--James D. Nichols, brother of prime suspect Terry L. Nichols--may have to be released next week because the explosives possession crimes he is accused of do not usually warrant holding someone without bail. To continue holding him, the government must convince a federal judge that there is a real risk he will flee.

“We have a lot to go on, and nobody’s talking about quitting, but we need a break,” a source close to the investigation said.

Perhaps the clearest indication of how hard investigators are working to squeeze investigative gold from unpromising gravel is the way they are dealing with Steven G. Colbern, the biochemist-turned firearms fanatic and survivalist who had been living in a cave near Kingman, Ariz., around the time that chief bombing suspect Timothy J. McVeigh was living there.

The UCLA-educated Colbern, who was arrested as a fugitive on federal weapons charges, kept a detailed diary, government sources revealed Wednesday. The diary contains admiring references to a “Terry Tuttle,” which is one of the names McVeigh used as an alias in the Kingman area, but it makes no mention of Oklahoma City.

Authorities are not sure what, if any, personal relationship existed between Colbern and McVeigh, but they hope the diary will yield clues to the identities of others who may have been associated with McVeigh in the relatively small circle of government-hating extremists in Kingman.

As a result, officials have brought additional charges against Colbern--based on his effort to resist arrest when captured last weekend--in what sources said was an attempt to persuade him to cooperate. The new charges could be bargained down or dropped entirely if Colbern proves helpful with the bombing inquiry, sources said.

Kingman is of particular interest because McVeigh’s residence there included the period last fall when he allegedly turned “clandestine,” using aliases, making extensive use of pay telephones and instructing relatives and associates to burn correspondence.

Officials acknowledged that their handling of Colbern amounted to legal hardball. But no more so than “the other aspects of the investigation” into the bloodiest act of terrorism ever on U.S. soil, one source said.

Sources close to McVeigh, who are as insistent on anonymity as those in the government, say the reason authorities have not advanced beyond two prime suspects is that no larger conspiracy existed.

The suspects’ military backgrounds and their experience with explosives after they left the Army means they would not have needed help planning an attack, making the bomb or delivering it. That is why investigators cannot find a John Doe No. 2 or other collaborators, those sources say.

One government source conceded that “it is not beyond the realm of possibility” that the bombing started and stopped with McVeigh and Terry Nichols, “but that’s not what our guts are telling us.”

McVeigh’s lawyer, Stephen Jones, played down the value of information attributed to sources in touch with his client, who is being held at the federal correctional institution in El Reno, Okla.

“We seem to have gone from extreme to extreme,” Jones said on NBC’s “Today” show. “The report for several days was that all he said was his name, rank and serial number, and now we have a report that he’s claimed responsibility. I don’t think we should place too much reliance on it.”

Jones was referring to a New York Times report that McVeigh had claimed responsibility for the bombing, that the Murrah building was chosen as a target because it housed so many government offices and was more architecturally vulnerable than other federal buildings. In a report based on two people who the newspaper said had talked with McVeigh in jail, McVeigh also is said to have expressed surprise to learn that children had died in the bombing and that he did not know the building included a day care center.

As a measure of the intensity of the investigation, the FBI has received more than 16,000 calls on an 800 number set up to draw information from the public and has entered 66,000 telephone numbers in its investigative database, officials said.

The numbers collected from calls linked to McVeigh and Terry Nichols and lesser suspects are identified and then matched by computers to see which numbers had contacted other numbers in the database.

Ostrow reported from Washington and Serrano from Oklahoma City.