Officials: Bin Laden guiding plots against U.S.
Ridge: Terrorists' aim is to influence presidential vote
Are press conferences on terror warnings politically inspired?
Test program for 'trusted traveler' system begins.
Tom Ridge: Al Qaeda may time attack to disrupt U.S. election.
WASHINGTON (CNN) -- A plot to carry out a large-scale terror attack against the United States in the near future is being directed by Osama bin Laden and other top al Qaeda members, senior intelligence officials said Thursday.
Bin Laden and his top lieutenant, Ayman al-Zawahiri, are overseeing the attack plans from their remote hideouts somewhere along the Afghanistan-Pakistan border, according to senior intelligence officials.
"This type of plotting, this type of operational activity, is being done with the direct direction and authorization of that senior leadership," said one official, who spoke on condition of anonymity.
A Democratic senator who attended Thursday's CIA and FBI briefing said, "It is the most worrisome situation since 9/11" without elaborating specifically.
Arrests of terror suspects in Europe and the Middle East resulted in the new warning, said Homeland Security Secretary Tom Ridge.
"We lack precise knowledge about time, place and method of attack," he said. "But along with the CIA, FBI and other agencies, we are actively working to gain that knowledge."
A senior U.S. intelligence official said the warning was based on "a very strong body of intelligence."
The planned attack is "an effort to disrupt the democratic process" before November's elections, Ridge said.
Ridge said he had no specific or credible information about threats to the upcoming political conventions. The four-day Democratic convention kicks off July 26 in Boston, Massachusetts, and the Republican National Convention begins August 30 in New York City.
He also said the nation's color-coded terrorist threat level will remain at yellow, or elevated, he said.
"We have more protective measures in place at yellow today than we did six months or a year ago," Ridge said. "And there will be more put in place on a day-to-day basis every single day, for as long as the department exists."
"We know they have the capability to succeed and they also hold the mistaken belief that their attacks will have an impact on America's resolve," Ridge said.
Possible Madrid model of attack
Sources said al Qaeda is plotting an attack similar to the Madrid train bombings, which killed nearly 200 people just days before Spain's elections in March.
Spain's elections resulted in the ouster of a conservative government that had joined the United States in the 2003 invasion of Iraq.
The opposition Socialists had campaigned on a pledge to withdraw Spanish troops from Iraq.
"They are focusing on what they perceive to have been successful attacks in Madrid as far as the impact on the electoral process there and the outcome of that election," the official said. "The reporting that we are seeing, the information that we have, is tied to the different types of democratic processes here."
Another senior intelligence official said authorities are concerned about polling places being targeted during the elections, and they are trying to come up with a protection plan.
A third intelligence official said there is particular concern about possible attacks using backpack explosives -- as were used in Madrid -- or car or truck bombs to destroy bridges, tunnels or other targets.
In April, arrests were made in Canada and Pakistan in connection with an alleged plot to set off a large bomb in London, the official said. The first official said al Qaeda also remains interested in targeting locations it has struck in the past.
"There are strong indications that al Qaeda will continue to try to revisit past targets, those that they were able to attack as well as those they were unable to attack," he said. "In addition, there is intelligence that indicates that they are looking at various transportation systems."
Al Qaeda remains "very interested" in using aviation, he added.
"We know that it is a consistent focus of their efforts as we saw in 9/11 and despite the numerous security enhancements that have been made, al Qaeda continues to pursue capabilities that can use aircraft either as weapon or as targets," the official said.
Anti-terrorism efforts have degraded al Qaeda's abilities, but the organization remains strong because of its flexibility and adaptability, one official said.
Accusations of scaremongering
Senators received a closed-door briefing on the threat Thursday morning from CIA and FBI officials.
Majority Leader Bill Frist, R-Tennessee, said the intelligence was "very non-specific" and there was was "no reason for panic, no reason for paralysis."
But he told reporters that there has been "general intelligence that there may be an attempt to disrupt elections."
"This is not a major announcement, it's just a fact," Frist told reporters. "The reality is of increased risk here in the homeland over the next several weeks, the next several months."
Sen. Charles Schumer, D-New York, called the information "troubling" and "worrisome."
But with no plan to raise the threat level and only general information, some other Democrats privately have questioned whether the timing of Thursday's briefing -- two days after President Bush's presumptive challenger, Sen. John Kerry, announced his running mate -- was politically motivated.
Ridge said any such suggestion was the "wrong interpretation." "We are basically laying out before the general public the kind of information that we have received, and these are not conjectures or mythical statements we are making," he said. "These are pieces of information that we could trace comfortably to sources we deem to be credible."
Another senior administration official said accusations of scaremongering are to be expected. But the official added, "This is one of those damned if you do and damned if you don'ts, and our default is 'do.' "
Kerry has accused the Bush administration of shortchanging homeland security since al Qaeda's September 11, 2001, terror attacks.
Kerry foreign policy adviser Randy Beers said the administration has been "unable to set the most elementary requirements for protection."
"Our crucial intelligence and military resources are overstretched abroad, and our homeland security effort at home is under-funded and poorly managed," Beer said in a written statement.
Schumer said Ridge's announcement shows the need for the Senate to move quickly on the $33 billion Homeland Security appropriations bill, which he and other senators said is being delayed by the Senate leadership in favor of a bill that would limit jury awards in state courts.
"We need that bill passed," Schumer said. "New York City is doing a great job on homeland security, but their resources are stretched as far as they can be stretched, and we're not getting the help from Washington that we need."
New York Police Commissioner Ray Kelly said the police department has been aware of a potential threat to the Republican National Convention and continues to take precautions.
"There is no new information," Kelly said. He explained that Ridge was acting in his role to raise public awareness of potential attacks.
"We do not think another attack is inevitable. But we do think that they will try. Our job is to stop them," Kelly said.
Operations center opens
Also Thursday, the Department of Homeland Security officially opened its state-of-the-art operations center, which will monitor real-time threat information and disseminate it to government agencies as well as state and local governments.
The Homeland Security Operations Center will operate 24 hours every day of the year in Washington, to monitor threats, share information and coordinate responses to incidents.
All 50 states are connected to the center through a secure, Web-based Homeland Security Information Network.
The center was scheduled to open in five months, but its opening was expedited because of general concerns about terrorism, a Homeland Security official said.
CNN Homeland Security producer Mike Ahlers contributed to this report.