THE arrest May 3 of two witnesses in the Oklahoma City bombing underscores both the triumphs and trials of what may be America's largest-ever manhunt.
In FBI offices in Oklahoma, Kansas, Arizona, and Michigan, agents have been working grueling hours since the bombing. From the command post at the Myriad Convention Center here, lawenforcement officials have been choreographing federal, state, and local agents in an elaborate nationwide crime-chasing dance that is, at once, both sophisticated and simple.
As some agents sift through the dust of the Alfred P. Murrah Federal Building looking for fragments of the fertilizer bomb or Ryder truck it was carried in, other agents are banging on doors from Oklahoma to Arizona, carrying sketches of ''John Doe No. 2'' to every gasstation attendant and hotel clerk.
Police divers plumb the depths of a Kansas swamp looking for explosives paraphernalia, while other agents, in sterile labs across the country, test fertilizer for ''fingerprints.''
The two witnesses detained yesterday, Gary Allen Land and Robert Jacks, were picked up at a motel in Carthage, Mo., 200 miles northeast of Oklahoma City. Mr. Land is reportedly a friend of bombing suspect Timothy McVeigh and resembles John Doe No. 2. Land and Mr. Jacks stayed for five months at a motel in Kingman, Ariz., a town frequented by Mr. McVeigh.
Of the tens of thousands of leads the FBI has received, many have been helpful. Far more haven't. On one occasion, FBI officers responded to a tip about John Doe No. 2 and wound up surrounding a Kansas motorist with a helicopter and a phalanx of police cars only to determine, after two hours of questioning, that he was just a motorist.
Some FBI officials contend privately that flagrant media reports about breaks in the case both complicate the investigation, and fuel unreasonable public expectations. For instance, while press reports said three witnesses had identified bombing suspect McVeigh in a lineup, FBI investigators testified before a federal magistrate that none of them, in fact, had been able to immediately identify McVeigh as the tattooed, crew-cut man they saw in a Ryder truck moments before the bombing.
Like everything else connected with the Oklahoma case, federal lawenforcement authorities are being closely scrutinized for their conduct. Certainly they have improved their image since the days of the Branch Davidian raid two years ago. Everyone from President Clinton on down has lauded agents for moving with dispatch and discipline.
Yet, as if a reminder of how difficult such cases can be, a serial killer known as the ''Unabomber'' -- who has claims to have eluded authorities for 17 years -- recently sent a letter to a major newspaper, ridiculing the FBI as ''a joke.''
Indeed, the biggest battle investigators may face is a public used to TV cop shows. ''People have been spoiled,'' says Harvey Burstein, a former FBI agent who now teaches a criminology course at Northeastern University in Boston. ''They seem to think that investigations can be wrapped up in 60 minutes with time for commercial breaks.''
The FBI is building a case one lead at a time. If someone calls the tip line and it sounds rational, Mr. Burstein says, ''the FBI is bound to check up on that lead, even if it goes nowhere.'' Above all, he says, investigators have to block out the public and remain detached.