Ashcroft said the intelligence, coupled with recent public statements attributed to al Qaeda, "suggest that it is almost ready to attack the United States."
"This disturbing intelligence indicates al Qaeda's specific intention to hit the United States hard," Ashcroft said.
The new terror warning comes just six months before the U.S. fall presidential election.
Some officials believe al Qaeda's goal is not to aid one candidate over another so much as to show it can influence voters, reports CBS News Correspondent Jim Stewart, just as it did in Spain earlier this year when bombs demolished commuter trains and toppled a prime minister who had backed the U.S.-led Iraq war.
The intelligence does not contain specifics such as timing, method or place of an attack. But officials say it is highly credible and backed with greater corroboration than usual, including information that operatives may already be in the United States.
Ashcroft, appearing with FBI Director Robert Mueller, drew new attention to photos of seven suspected al Qaeda operatives that the FBI has been pursuing for months.
Gadahn is an American citizen. Siddiqui is a woman.
"They all pose a clear and present danger to America," Ashcroft said.
Mueller said that "extraordinary precautions" already were being taken to protect the sites of the two political conventions — the Democratic convention in Boston in late July and the Republican convention in New York in late August — as well as next month's Group of Eight economic summit on Sea Island in Georgia.
Some law enforcement and firefighter union representatives, supporters of Democrat John Kerry for president, suggested that the timing of the threat report was suspicious because of polls showing a sagging approval rating for President Bush. International Association of Firefighters President Harold Schaitberger told reporters in a conference call that the intelligence has been in the government's hands for weeks.
White House press secretary Scott McClellan, however, denied that there is a political aspect to the threat report.
"The president believes it's very important to share information appropriately," McClellan said. "We do that in a number of ways when it comes to looking at the threats we face here in the homeland."
Ashcroft said the Justice Department was seeking "unprecedented cooperation" from state and local law enforcement and was offering "unprecedented access" to its intelligence information
To focus on the threat, the FBI has established a 2004 Threat Task Force and FBI analysts are reviewing previously collected intelligence to see if it contains any clues to the latest threat. There will also be a series of interviews conducted by the FBI with individuals who could have information about potential plots.
The FBI was dispatching a bulletin to some 18,000 state and local law enforcement agencies warning of the threat.
However, there are no immediate plans to increase the U.S. terror alert level.
On Wednesday morning, Homeland Security
"Every single day it's our job within the Department of Homeland Security to work with federal, state, local officials in the companies in the private sector to get smarter and more secure, Ridge said. "We don't need to raise the threat level to bring together some of the best and brightest minds in this country to use people and technology to be
Ridge said people should "go about living their lives as Americans. We'll provide the security, they have to figure out how to have fun themselves."
Along with the high-profile events this summer, the FBI and Homeland Security Department also are concerned about so-called soft targets such as shopping malls anywhere in the United States that offer a far less protected environment than a political convention hall.
Of special concern, the counterterrorism official said, is the possibility that terrorists may possess and use a chemical, biological or radiological weapon that could cause much more damage and casualties than a conventional bomb.
While the government has worried for years that al Qaeda may try to get its hands on such weapons, CBS News has learned that it is a recent bomb plot in Saudi Arabia that has them truly worried. The plot illuminated a new tactic to attack large fortified public-like buildings.
Terrorists planned to use two truck bombs — one to blow up outside defenses, and a second to drive through the opening — and take down the building.