The disclosure that the Federal Bureau of Investigation withheld documents from Timothy McVeigh's lawyers seems certain to ignite a controversy that will burn for years, perhaps decades. ''If any questions or doubts remain about this case, it would cast a permanent cloud over justice,'' said Attorney General John D. Ashcroft in delaying Mr. McVeigh's execution until at least June 11. But for some people the cloud has been there all along, and always will be. They will never accept the government's assertion that the withholding of the documents was simple human, bureaucratic error. And so the 1995 bombing of a federal office building in Oklahoma City seems likely to join the assassinations of John F. Kennedy and the Rev. Dr. Martin Luther King Jr. as events whose truth -- in the eyes of some Americans -- is forever untold. ''Gee, how did that happen?'' Charles Key, a former member of the Oklahoma House of Representatives, said in sarcastic response to the news that the McVeigh defense team had been denied evidence held by the government. Mr. Key, who lives in Oklahoma City, said he has always been convinced that Mr. McVeigh had accomplices beyond Terry Nichols, the other person convicted in the bombing. The government has not pursued the case aggressively, he said, because the full truth would be too damning.