OKLAHOMA CITY, Aug. 7— A new mystery surfaced in the Oklahoma City bombing today, just days before formal charges were to be lodged, when the State Medical Examiner disclosed that a severed leg and foot clad in a combat boot had been found deep in the rubble. 
The brief announcement, more than two months after the authorities said all victims were accounted for, came after the lawyer for Timothy J. McVeigh, the prime suspect in the bombing which killed 167 people, said he would ask prosecutors to explore information that a severed leg had been found that could not be matched to a body. 
The lawyer, Stephen Jones, speculated that the leg and foot may have belonged to "the real bomber" who could have been blown apart in the April 19 explosion. 
The finding, which raises the possibility of there being another participant, came as the deadline neared for Federal prosecutors to bring indictments for the Federal Building blast against Mr. McVeigh and another Army veteran, Terry L. Nichols. A third member of the same Army unit from Fort Riley, Kan., Michael Fortier, is also likely to be indicted, although he has been negotiating for leniency in exchange for his testimony and may face lesser charges. 
Mr. Fortier's wife, Lori, was brought to the Federal Courthouse here today for a 15-minute meeting with a judge in which she was granted immunity in exchange for her testimony before the grand jury, expected on Tuesday. The grant of immunity for Mrs. Fortier was a chief element in the deal that Mr. Fortier was attempting to strike with the Federal prosecutors. 
But the existence of an unknown body could cast the case in a new light. One possibility is that the leg belonged to the second man some witnesses reported seeing with Mr. McVeigh in the period just before the bombing but whom investigators have never been able to identify. Thus far, despite a huge search for a "John Doe No. 2," Federal investigators have not identified any other participants. 
The Medical Examiner's office, in a statement issued today, said it was continuing to analyze unidentified human tissue recovered from the Federal Building, among which was "a traumatically amputated left thigh and lower leg recovered on May 30, 1995." 
"This leg was clothed in a black military type boot, two socks, and an olive drab blousing strap," the statement continued. "Anthropological analysis of this specimen reveals the individual to be light skinned, dark haired, probably less than 30 years of age, male (75 percent probability), and having an estimated height of 66 plus or minus three inches." 
The Medical Examiner's office said that the leg had not been matched to any other known victims or survivors. 
The operations director of the Medical Examiner's office, Ray Blackeney, said the severed leg was found after the last three identified bodies were removed from the building, an operation that was delayed until after the building was demolished because of fears that the structure would collapse. 
At the time of the recovery of the three bodies, Mr. Blackeney had dismissed the possibility that a bomber was killed in the blast, saying: "As far as I'm concerned, he's not there. We have found the people where we thought we would find them. We didn't find anybody we didn't think was there." 
Because the leg was found after the building was demolished by controlled explosions on May 23, it may be difficult to determine its original location. But because the area above the three bodies was thoroughly searched, and the wreckage sealed off and guarded, it would appear that the leg had been further down in the rubble, near the center of the blast. That location would be consistent with someone in or near the exploding truck. Further, Mr. Blackeney said the leg was the largest identifiable body part, suggesting that the rest of the individual had disintegrated from the force of a nearby explosion. 
The "olive drab blousing strap" mentioned in the Medical Examiner's report is a thin elastic band commonly used in the military to "blouse" trousers by tucking the bottoms around the strap and pulling them down over the tops of high combat boots for a neat appearance. 
The military personnel -- mostly recruiting officers -- inside the Federal Building were all accounted for. However, a veteran or someone who, like Mr. McVeigh, favored military garb might be likely to use blousing straps. 
Asked in a telephone conversation why the Medical Examiner's office had not made the existence of the severed leg public before, Mr. Blackeney replied; "Why should we?" 
"I knew about it," he said. "We all knew about it here at the Medical Examiner's." 
He said that the Medical Examiner's technicians had been working with the Federal Bureau of Investigation using DNA and other tests on the severed leg and other pieces of tissue in hopes of establishing identifications. 
"This has come out a little bit premature," Mr. Blackeney said. "This is the last piece of the puzzle and I hope at some point to offer an explanation."