In recent years, the government has told Americans it has credible evidence of impending terror attacks, of weapons of mass destruction in Iraq and of collaboration between Al Qaeda and Saddam Hussein. But "credible" doesn't mean the same thing to every government official and even credible information can be wrong.

The warning this week from Attorney General John Ashcroft (search) of a possible impending Al Qaeda (search) attack in this country has set off a new debate among ordinary citizens, on the presidential campaign and inside the government over the meaning of "credible."

Even some intelligence professionals were confused and suspicious of possible political motives for the announcement because there were no specifics about time, location or method of attack and no new information about the seven terror suspects. They also noted the Homeland Security Department did not raise the national alert level from yellow, midpoint on the five-color scale, to orange.

Al Qaeda's intent to launch a major attack in this country has been no secret for some time, former CIA counterterrorism chief Vince Cannistraro noted Thursday. "The question is their capabilities, and that we don't have."

"Nobody is telling us details of an operation except people talking openly on the Internet, who know we are reading it," Cannistraro said. "If al Qaida is planning another 9/11, do you think they'll tell us on Web sites? Give me a break."

Retired FBI field office chief C.I. Smith, who spent most of his 25 years in counterintelligence, said, "I was perplexed by the timing. ... If it was truly credible, you should have also had [Director George] Tenet from CIA and [Secretary Tom] Ridge from Homeland Security, too."

An administration official said Ashcroft's news conference started out as a request for the public to be on the lookout for seven suspects and an announcement of stepped-up security efforts for high-profile events this summer. Instead, it turned into a major news story that the official said included references to new intelligence. The official requested anonymity to avoid causing friction between Homeland Security and Justice.

U.S. intelligence officials have said credible information suggests terrorists may want to target events this summer, but they also note U.S. authorities have been concerned about that threat for some time.

To professional intelligence analysts, credibility grows out of history. "Specific credible intelligence" of such an attack would mean details of plans from a source, either human or electronic, that had provided reliable information in the past, Cannistraro said Thursday. He didn't see any evidence of that in Wednesday's announcement.

Ashcroft "presented it as though he was tracking this minute by minute when he said the preparations have gone from 70 percent to 90 percent complete," Cannistraro said, who called that bunk.

In addition, analysts would try to determine whether the informant had access to such data and what his motivations might be - revenge, money, ideology or whatever, said Larry Mefford (search), former FBI counterterrorism chief. Then analysts try to see if all or parts of the information can be corroborated or fit with other known events.

But that makes it sound easier - and more clear - than it is.

Author Bob Woodward writes in a new book that National Security Agency director Michael Hayden tried to explain to his wife the difference between the black-and-white world of facts and the gray world of intelligence this way: "If it were a fact, it wouldn't be intelligence."

Sometimes intelligence isn't discovery of a missile but an attempt to peer into the mind of an enemy leader to learn when he will use it.

Inside the FBI, Mefford said, "`credible intelligence' is a term taken very seriously. It always referred to information we believed to be reliable."

Nevertheless "credible intelligence is a very subjective term," Mefford added. And the analysis of intelligence "is not a science; it's an art."

The FBI is still trying to improve the formal internal process by which it vets the reliability of sources, Mefford noted.

Recently, Secretary of State Colin Powell (search) conceded the Bush administration accepted distorted reports of Iraqi weapons of mass destruction. Other administration and congressional officials said the misleading information came from Ahmed Chalabi, a prominent exile who wanted to get the United States to overthrow Saddam so he could return to Iraq.

In January, departing U.S. weapons inspector David Kay (search) said he no longer thought such weapons existed in Iraq. He said it was clear by then that the CIA's problem was a lack of its own spies in Iraq who could provide credible information.

This March, however, Charles Duelfer, now the CIA's supervisor of the search, said his group regularly receives reports - "some quite intriguing and credible" - about possible weapons stashes hidden in Iraq. But if these informants are judged credible because of previous reliable reports, such earlier reports must be about something other than caches of weapons of mass destruction because none of those have been found.

This debate also has played out in congressional hearings. The CIA's Tenet told the Senate Armed Services Committee that policy-makers are entitled to flexibility in describing intelligence and don't use precisely the same words as intelligence analysts.

But Sen. Edward M. Kennedy (search), D-Mass., angry over what he saw as Bush administration exaggeration of the justification for the Iraq war, replied, "I'm not talking about parsing words. We're talking about words that are basically warmongering."