Federal officials exploring links between two brothers and the Oklahoma City bombing said they plan to charge both James and Terry Nichols with explosives and firearms violations after extensive searches of their properties. Sources said Terry Nichols also might soon be charged as an accomplice in the bloodiest terrorist act in American history.

Investigators recovered explosive materials at James Nichols's 500-acre farm outside Decker, Mich., and at Terry Nichols's home in Herington, Kan., according to senior law enforcement sources. The evidence was said to closely match the materials used in the bombing, although the sources cautioned that at this point the explosives charges will be only tangentially linked to the larger bombing case.

Agents also recovered several empty or nearly empty 55-gallon drums used to hold fertilizer, caps and igniters used with explosives and at least two dozen firearms, including a fully automatic Uzi-style assault pistol, from Terry Nichols's property. The Nichols brothers are being held as material witnesses in the case.

The evidence recovered from the brothers, one senior law enforcement source said, convinced authorities that the investigation was "on the right track." At least four men are believed to be involved in the bombing, including the still-at-large suspect known as John Doe No. 2, the tattooed and square-jawed man who was seen renting the truck used in the bombing in the company of the prime suspect, 27-year-old Timothy James McVeigh, who was arrested last week.

Early yesterday morning, federal agents in California apprehended a man they at first believed might be John Doe No. 2 but later decided was not. He was an Army deserter named David Delgado Iniguez who went AWOL in late August from Fort Riley, Kan., the same military base where McVeigh had been stationed during his stint in the service. Although ruling out Iniguez as the John Doe suspect, they nonetheless detained him because of his possible connection to McVeigh. Iniguez, who was picked up in a working-class enclave on the edge of San Bernardino, apparently shared McVeigh's interest in paramilitary groups, but their connection beyond that was unclear.

Senior federal law enforcement sources said they had intensified the manhunt for the John Doe accomplice and other possible suspects. They were working, they said, from a list of men who both fit the John Doe description and were known to have associated with McVeigh, who has been booked on capital charges in connection with the bombing, or with the Nichols brothers.

The central focus of the widespread probe is on men who are believed to have ties to far-right paramilitary groups that share a deep mistrust of the federal government in general and federal law enforcement officials in particular. Agents were concentrating much of their attention on three places: Michigan, where the Nichols brothers lived and associated with the paramilitary Michigan Militia; Kansas, where both McVeigh and Terry Nichols served in the Army and where McVeigh and John Doe No. 2 conducted most of their pre-bombing activities; and Arizona, where McVeigh lived recently and where he associated with a paramilitary group known as the Arizona Patriots.

McVeigh's activities in Arizona seemed especially intriguing to federal officials. For several months in 1994 he lived in a trailer park on the edge of Kingman, a desert town along Route 66 on the western side of the state. The region, according to federal sources, in recent years became a haven for paramilitary groups attracted to its isolation and the lack of gun laws. Federal sources said there have been several mysterious explosions in the vast desert expanses near Kingman, including one a few months ago in which the materials used matched those in the Oklahoma City blast.

McVeigh's connections to the Arizona militia groups is still uncertain. Sources said many of his close friends there were former servicemen who were interested in guns and militia organizations. At the two places where McVeigh worked during his stay in Kingman -- a hardware store and another company where he served as a security guard -- he always wore Army fatigues.

Residents of the trailer park where he lived said he was difficult. The landlord constantly complained that McVeigh played rock music too loud and failed to keep the yard clean. When he was told he was breaking park rules, manager Bob Ragin said, McVeigh responded in "paranoid" fashion -- "as though he was the only one being singled out." Danny Bundy, 30, a plumber who lived next door to McVeigh, said the former serviceman constantly wore his military fatigues at home. "He would go around with his camouflage pants and without a shirt and would walk around like he thought he was pretty mean," Bundy said.

A security guard who worked with McVeigh recalled an odd experience once when they went to a shooting range. Fred Burkette said that the two took target practice for three hours and that just as they were about to walk off the range, McVeigh went on a rampage. "He just started shooting like he was just trying his best to use up 200 bullets," Burkette said. "First he was aiming at a tree, then a fence post. When he got through he had almost cut it in half. He was hitting lots of targets -- to me that was off the wall."

McVeigh also used to talk to Burkette about his anger over federal involvement in the Waco incident. In the affidavit filed by the FBI in connection with McVeigh's arrest, federal authorities stated McVeigh had been a spectator outside the Branch Davidian compound in Texas while it was under a 51-day siege by federal agents and was "extremely angry" about the federal role in the confrontation, in which more than 80 cult members and four federal agents were killed. "He mentioned that the FBI had no right to do what they did, that they had broken the law," Burkette said.

Authorities found correspondence in McVeigh's car vowing revenge on federal agents for Waco, the Dallas Morning News reported. It quoted federal sources as saying witnesses identified McVeigh in a lineup and said he had asked directions to the federal building just before the explosion.

As investigators increased their focus on Terry Nichols after explosives and weapons were found in his Kansas home, more details emerged about his life and recent past. Nichols and his brother James grew up in Lapeer, Mich., and he spent many years, before and after his stint in the military, farming outside Decker. But by the time Nichols left Michigan for Kansas last year, he was an extremely troubled man. His first marriage had failed. His 2-year-old son had died from suffocation -- a garbage bag over his head -- in an incident that police ruled accidental. He had been tangled in a range of local legal battles over defaulted loans and property disputes.

"He and James always seemed to be trying to beat the system," said Daniel Stomber, a longtime neighbor. "But they were like two sides to the same coin. James would not shut up and Terry never really said anything." When Terry Nichols did talk, he often derided the government or authority figures of any sort, Stomber recalled.

In petitioning the local courts in cases involving loan defaults and a credit card bill, Nichols lambasted his own lawyers, writing: "If all these bloodsucking parasites disappeared, this whole world would be better off." During some proceedings, Nichols refused to address the court in the normal fashion and instead shouted answers to the judge from the rear of the courtroom.

His disgust with the government was most obvious in August 1992, when he sent a letter to the clerk in Evergreen Township in which he returned his voter registration certificate, stating: "There is total corruption in the entire political system from the local government on up through and including the President of the United States of America, George Bush."

Another way that both Terry and James Nichols showed their contempt for government, neighbors said, was by defacing dollar bills with an ink stamp. Friends of James Nichols said he boasted the bills would have to be taken out of circulation once they reached a bank.

The Nichols brothers attended at least a few meetings of the Michigan Militia over the last two years, according to neighbors and members of the group. At least once, they are said to have taken McVeigh along. The militia, which claims a membership of more than 10,000, expresses passionately anti-government views, but its leaders have said in recent days that they considered the Nichols brothers extreme and once kicked one of them out of a meeting.

According to federal sources, a few members of the Michigan paramilitary outfit had been under investigation recently, before the Oklahoma bombing. One incident that may be related to the militia and has federal officials puzzled involves a message dispatched on the morning of the bombing. Freshman Rep. Steve Stockman (R-Tex.), a staunch opponent of federal gun laws, received a fax containing cryptic information about the bombing as though it were a report from the scene. The fax had the word "Wolverine" on the top and was traced to a Michigan area code. Wolverine is a small town in northern Michigan near militia headquarters. Federal officials yesterday were seeking a Michigan Militia leader, Mark Koernke, for questioning in connection with the mysterious fax.

New information also emerged yesterday from the Kansas part of the bombing probe. Kansas represents the link between McVeigh and Terry Nichols. They met there, at Fort Riley, and Nichols lived in Herington in recent months at the same time McVeigh was said to be preparing the bombing operation in nearby Junction City. The search of Nichols's Herington home led federal agents to believe that the two men had been in recent contact, according to one source.

In the days before the bombing, McVeigh rented a room at the Dreamland Motel near Fort Riley. According to the owner, Lea McGown, he stayed there from Friday, April 14, until last Tuesday, the day before the bombing. He stayed in Room 25, next to the office. McGown said McVeigh arrived in a yellow 1977 Mercury sedan but a few days later came back with a Ryder truck that he parked at the end of the parking lot far from his room. The management told McVeigh to move the truck because it was too close to a ditch.

Interviews with a local deliveryman yesterday added a new description to McVeigh's companion during his stay at the Dreamland. Jeff Davis, who delivers for the Hunan Palace Chinese restaurant, said he dropped off an order of moo goo gai pan and an egg roll to Room 25 on Saturday night. The man who answered the door, Davis said, was definitely not McVeigh, but had "longer hair and a fuller face." The man who requested the food gave the name Bob Kling, the same alias used when McVeigh and his alleged accomplice rented the truck.

In their search of Nichols's Herington property, authorities found the fertilizer drums and caps and igniters used with explosives. Neighbors told police they were impressed with Nichols's use of fertilizer to help his lawn, sources said.

Nichols came under suspicion late Thursday or early Friday, and federal agents put him under surveillance, sources said. He turned himself in to the Herington Police Department around 3:30 p.m. Friday. The FBI arrived minutes later and questioned him in a basement room until 1 a.m., when he was arrested as a material witness. Sources said Nichols admitted knowing McVeigh from Fort Riley, but investigators were frustrated because he would not say much more during the heated sessions. This story was reported by staff writers Thomas and Maraniss in Washington, Serge F. Kovaleski and Rene Sanchez in Decker, Mich., and Nell Henderson in San Bernardino, Calif., and by special correspondents Thomas Heath in Fort Riley, Kan., and Kathryn Wexler in Kingman, Ariz. CAPTION: Explosives experts from Fort Riley, Kan., were called in to search the Herington home of Terry Nichols. FBI agents prepared a robot Saturday. CAPTION: Above, Army deserter David Delgado Iniguez, his face obscured by a towel, was detained in California because of his possible links to suspect Timothy James McVeigh. At right, James Nichols, face also obscured, leaves a Michigan county jail on his way to Oklahoma for questioning in the bombing.