Federal authorities yesterday formally charged Terry Lynn Nichols and his brother James with conspiring with Timothy James McVeigh to build explosives at their farm in Michigan, while investigators compiled new evidence linking McVeigh to the terrorist bombing in Oklahoma City.

The charges against the Nichols brothers are officially unrelated to the bombing but will allow federal authorities to keep the men in custody while they pursue other leads connecting them to McVeigh, who is being held in an Oklahoma prison on bombing charges. He is refusing to answer questions, telling interrogators he considers himself a "prisoner of war."

As the pace of the massive federal investigation quickened six days after the worst domestic terrorist attack in U.S. history, the FBI also released a more detailed composite sketch of another man sought in connection with the blast -- identified only as John Doe No. 2. He remains at large and is the subject of an intensive nationwide manhunt. The FBI says it has received some 7,000 credible leads in the case.

Forensic experts meticulously sifting through rubble in the blast crater have found fragments of gas cylinders that investigators believe were used to boost the explosive power of the device. "They were intent on killing," said one law enforcement official, noting that the cylinders, which possibly contained hydrogen or acetylene, suggest the 4,000-pound bomb was constructed and detonated only after careful and deliberate planning.

Residue from bombmaking materials was found on McVeigh's clothing and in the yellow Mercury sedan driven by the 27-year-old former Army infantryman when he was picked up on a traffic charge shortly after last Wednesday's explosion. The residue is ammonium nitrate, which authorities say was mixed with fuel oil to concoct the devastatingly lethal bomb.

Investigators also have found a note, apparently discarded in a police patrol car by McVeigh, that investigators say refers to a request for obtaining explosives.

The FBI's new sketch of John Doe No. 2, based on the recollections of witnesses who say the two men rented a Ryder truck in Junction City, Kan., shows the by-now familiar square-jawed face topped with a new detail: a cap with two stripes.

In Junction City, the manager of the Great Western Inn was watching television when the new sketch was broadcast. He said it closely resembled the driver of a Ryder truck who stayed at the motel April 17, two days before the bombing. Federal authorities believe the Oklahoma City bomb was transported in that truck. FBI agents have searched the motel room and taken records from the office. McVeigh also stayed in Junction City at a different motel from April 14 to April 18, the day before the bombing.

The motel manager, who asked not to be identified, said the truck driver used a foreign name and spoke broken English. The man said he was from Colorado and paid his $29.90 bill with a travelers check. The manager described the man as 5 foot 10 or 5 foot 11, weighing 190 to 200 pounds, with a mustache, long hair and light brown complexion. The FBI sketch does not show a mustache. Motel employees also said someone identifying himself as James Nichols III, with a Georgia address, stayed in the hotel around April 7 or 8.

In Milan, Mich., U.S. Attorney Saul Green filed formal criminal charges against Terry Nichols, 40, McVeigh's friend and fellow infantryman, and Terry's older brother, James, 41. The brothers have been in custody since Friday as material witnesses -- Terry in Kansas and James in Michigan. At yesterday's hearing, James Nichols was ordered held without bond until Friday. His brother has a hearing scheduled Wednesday in Wichita.

According to the complaint, the Nichols brothers from 1992 to 1995 conspired together and with others, including McVeigh, to "make and possess firearms, that is destructive devices." Authorities seized 28 50-pound bags of fertilizer containing ammonium nitrate, a 55-gallon drum of fuel oil and large quantities of 35 percent hydrogen peroxide at James Nichols's Decker, Mich., farm. "All these materials can be used in ingredients in improvised explosives," stated the complaint. Also seized were nonelectric blasting caps and black powder.

An FBI affidavit filed in Michigan provided new information on McVeigh and the brothers. It quoted James Nichols as saying he had seen his brother and McVeigh making crude "bottle bombs" from materials such as gasoline and brake fluid as early as 1992 at his home. "James Nichols further stated that he believed that Timothy McVeigh had the knowledge to manufacture a bomb," said FBI Agent Patrick Wease in the document.

The affidavit also referred to the brothers' frequent diatribes against the federal government, in particular their denunciations of the 1993 raid on the Branch Davidian compound in Waco, Tex., in which more than 80 people died. James Nichols, said the FBI agent, "made comments stating that judges and President Clinton should be killed and that he blamed the FBI and {Bureau of Alcohol, Tobacco and Firearms} for killing the Branch Davidians in Waco." The FBI later issued a revised affidavit removing the reference to Clinton without explanation.

With evidence mounting, federal officials are developing a more coherent and detailed theory of how the bombing was executed. They believe at least four people were involved and that Oklahoma City may have been targeted because some of the FBI and ATF agents involved in the Waco siege were headquartered there.

They believe the components of the bomb may have been purchased and combined in Kansas, then driven to Oklahoma City in the Ryder truck. The truck was parked in front of the Alfred P. Murrah Federal Building just before 9 a.m., abandoned by its driver and detonated within minutes, either by a timing device or remote control. The driver presumably escaped in a car parked nearby. Three witnesses have placed McVeigh in front of the building shortly before the blast. They say no truck was visible at the time, suggesting that McVeigh was waiting for the bomb-laden truck to arrive.

The first breakthrough in the case came when investigators recovered a vehicle identification number from an axle from the truck. They traced the vehicle to the Ryder outlet in Junction City, where employees gave them descriptions of the two men who rented it. After issuing composite sketches of the men, agents discovered that McVeigh was in jail in Perry, Okla., where he had been stopped on a routine traffic violation less than 90 minutes after the bombing and was discovered to have a gun in his possession.

Investigators now believe McVeigh may have driven the getaway car on the morning of the bombing. They found a handwritten note in his yellow sedan stating the car was having mechanical difficulty -- suggesting the 1977 Mercury could have been parked near the federal building with the note in the window so that it would not be towed.

In an attempt to break McVeigh's silence, federal authorities are considering flying in his father, William, from western New York to talk to him. Investigators also zeroed in on McVeigh's sister, Jennifer, searching her Pensacola, Fla., residence for explosive devices, right-wing literature and correspondence with her brother, and they have questioned her. McVeigh's mother, who also lives in Florida, issued a note expressing sympathy for the victims and pleading for the media to "please leave our family alone."

McVeigh's Arizona driver's license listed James Nichols's Michigan farmhouse as his home address, which led investigators to the property and to the discovery of the explosive materials that led to yesterday's charges. Terry Nichols and McVeigh served in the same infantry unit together, and McVeigh worked at the farm for a time after his discharge from the service in 1991. Terry Nichols and McVeigh shared a fascination with explosives, firearms and rabid anti-government sentiments. Terry Nichols later moved to Kansas, buying a small house in Herington.

Neighbors and acquaintances in the small town near Lake Huron described the Nichols brothers as having an abiding rage about government, particularly about taxes. James Nichols in particular drew the notice of townspeople with his car plastered with anti-government bumper stickers and his penchant for defacing dollar bills.

But a report that McVeigh had left a pregnant girlfriend and a trail of rude, even paranoid behavior at a Kingman, Ariz., trailer park where he once lived turned out to be false. Trailer park owner Bob Ragin said he had mistaken McVeigh for another person and said the ex-serviceman had in fact been a model tenant -- "extremely polite and well-behaved."

A Florida newspaper reported that McVeigh attended a meeting of about 250 militia members in St. Lucie County last year with Mark Koernke of Dexter, Mich., whose short-wave radio program and videotape sales have made him a national figure among far-right militia groups. Quoting local militia members, the Tribune of St. Lucie County reported that McVeigh and seven other men, including Koernke, drove a van down from Michigan to attend the two-day meeting at a ranch in rural Florida. McVeigh, militia members told the Tribune, was one of Koernke's several bodyguards.

Kenny Kirkland, spokesman for the local militia regiment, said McVeigh went to the meeting because "he heard the Branch Davidians, the people who survived Waco, would be here." Kirkland said when he saw a television broadcast of McVeigh being arrested in the Oklahoma bombing, he recognized him "instantly" from the gathering at the ranch.

Koernke told reporters Monday that he did not know McVeigh or the Nichols brothers and insisted he had nothing whatsoever to do with the bombing. FBI agents searched his Wolverine Productions office Monday and have said they want to question Koernke about the men under arrest and about a cryptic facsimile that Koernke sent to about 200 people, including members of Congress, within an hour of the blast.

On Capitol Hill, meanwhile, a Senate resolution, adopted on a 97 to 0 vote, condemned the bombing and pledged to enact legislation to bolster the ability of federal agencies to combat terrorism. Also in Congress, a House subcommittee postponed a mark-up, scheduled for next week, of legislation repealing a ban on the sale of some semiautomatic assault-type weapons.

President Clinton will meet today with a bipartisan delegation of congressional leaders to discuss anti-terrorism legislation. Special correspondent Thomas Heath in Junction City, Kan.; staff writers Serge F. Kovaleski and Rene Sanchez in Decker, Mich., William Claiborne in Kingman, Ariz., and Susan Schmidt in Washington, and researcher Barbara J. Saffir contributed to this report. CAPTION: Above, Mark Price, a state officer with the Michigan Militia, signs autographs in Decker. Below right, from left, Sens. Don Nickles (R-Okla.), Joseph I. Lieberman (D-Conn.) and James M. Inhofe (R-Okla.) meet with reporters after the Senate condemned the bombing. CAPTION: At left, federal agents leave James Nichols's farm in Decker, Mich., on Monday after four days of searching for evidence in the bombing. Below, from left, Sens. Don Nickles (R-Okla.), Joseph I. Lieberman (D-Conn.) and James M. Inhofe (R-Okla.) speak to reporters after the Senate adopted a resolution condemning the bombing. (Photo ran in an earlier edition)