DENVER — An apparently half-baked plot by three Denver-area men to assassinate Sen. Barack Obama on the basis of his race has highlighted the question of exactly what and whom to fear as Obama becomes the first black presidential nominee of a major party.

"We're taking it very seriously," said Troy Eid, the U.S. attorney for Colorado, at a news conference on Tuesday. But he said the racist rants that the men made after their arrests on Sunday — all three for possession of methamphetamine, and two of them on weapons charges — had not risen to the level of a "credible threat" that would have allowed federal charges of threatening Obama to be filed.

"It was one meth-head talking to another about life," Eid said.

In Washington, FBI officials on Tuesday initially seemed to view the episode as relatively minor. But as the day wore on, officials acknowledged stronger concern about the men and their motives.

"Central questions remain unanswered," said a senior FBI official, who spoke on the condition of anonymity because the investigation is continuing. "Is there anyone else involved, and how real was this plot? Or was it just a discussion fueled by people with delusions of grandeur and too many drugs?"

An equally important question is whether that distinction really matters. If the men were tied to an organized white-supremacist group or were simply on their own — violent, drug-addled lowlifes who just happen to be racist — the stew of extremist paranoia and hate is still the same, several law enforcement officials and hate-crime experts said.

Officials in Washington said they were not planning any change in security and had already braced for the prospect of threats from white supremacists or extremist groups against Obama. Obama will speak Thursday before 75,000 people at INVESCO Field, where security is expected to be extra heavy.