Al Qaeda may try to attack the United States sometime this summer in an effort to disrupt the American political process, Homeland Security Secretary Tom Ridge (search) warned Thursday.

One potential target could be this year's presidential conventions. The Democrats will meet at Boston's FleetCenter July 26-July 29, and the GOP convention will be held at Madison Square Garden in New York City from Aug. 30-Sept. 2.

"Since Sept. 11, 2001, we have had intelligence that Al Qaeda (search) intends to launch more attacks against the homeland," Ridge said during a press conference. "Credible reporting now indicates that Al Qaeda is moving forward with its plans to carry forward a large-scale attack in the United States in order to disrupt our democratic process."

While officials don't yet have specific information on the time, place and method of attack, Ridge added, "we are actively working to gain that knowledge." The FBI has formed a special task force to respond to the convention threat.

"We are very comfortable with the credibility of the sources" of the information, he continued, but added that the information itself it still being corroborated.

Law enforcement officials told Fox News that these sources are independent of each other and that the threat information came from interdictions in England, Jordan and Italy.

Ridge said recent overseas terror arrests indicate that some Al Qaeda cells are operational and that the network has people in place and the materials to carry out attacks.

A political convention has "all of the elements of a terrorist's target — it's got a lot of people there, it's very symbolic … and it's also very political," former CIA operative and senior fellow at The Heritage Foundation, Peter Brookes, told FOX News. "The threat continues and there's no doubt this is a prime time to insert themselves — not only to kill Americans, but to insert themselves into our democratic process."

Key lawmakers received top-secret briefings Wednesday and Thursday by Ridge, FBI Director Robert Mueller, CIA Director George Tenet and other officials from the Department of Homeland Security and the CIA.

"I think we need to keep in mind first that Al Qaeda's aim is to kill Americans, their victory is defined as murdering innocent civilians," said Rep. Chris Cox, the California Republican who chairs the Select Committee on Homeland Security. "There is a steady drumbeat of reporting and the threat has not abated."

The goal of terrorists here, according to law enforcement, is an attack with Madrid-style implications. That attack in Spain on March 11 probably altered the election results in that country, as the U.S.-friendly former President Jose Maria Aznar (search) was ousted.

Ridge said DHS is ramping up security in some areas, such as chemical plants, and taking a hard look at trucks that Al Qaeda may use to launch a "vehicle-borne explosives." Transportation Security Administration officials told Fox News that they are paying special attention to foreign flight crews and pilots, fearing hijackers may try to infiltrate their ranks.

Ridge will visit the Democratic convention site in Boston as early as next Friday, and he will also meet the week of July 20 with officials from major league sporting organizations.

"We have briefed the campaigns, both campaigns — the Kerry-Edwards campaign as well as the Bush-Cheney campaign — about the security measures that are being put in place for those conventions in New York and Boston," said White House press secretary Scott McClellan.

NYPD: 'Our Job Is to Stop Them'

New York Police Commissioner Ray Kelly gave a press conference later Thursday in response to the security questions generated by Ridge's remarks.

"New York City remains at the second highest alert level, level orange," and has been there ever since Sept. 11, 2001, he said. "There are no plans to change that."

Also saying there's no specific information about a pending attack in the area, the NYPD has taken precautions over the past four months to boost security in light of intelligence that showed "Al Qaeda was emboldened by the Madrid bombings and may try to target the conventions."

After the Madrid bombings, which killed 190 people, the ruling party was voted out of power and Spain subsequently announced it would withdraw troops from Iraq.

NYPD detectives and intelligence officers went to Spain after those attacks to study the affects of the bombings and to familiarize themselves with the method.

"As a result, we reconfigured security in the [NYC] transit system overnight," Kelly said. "The notion that terrorists may attack during the convention has been part of our planning from the very beginning," including putting 10,000 police officers on duty during the conventions.

"We in New York are mindful of that fact that terrorists have struck [here] twice … we do not think another attack is inevitable but we do think they will try. Our job is to stop them."

Ridge: 'This Is Sobering Information'

Ridge said Thursday that while the national terror alert will not be raised right now, heightened security measures are being put into place.

"We have permanent protections in place today that did not exist a year ago … and these protections make it harder for terrorist to attack us," the secretary said.

The United States now has, for example, a fully-connected nationwide 24-7 homeland security operations center that electronically provides emergency and threat information to all 50 states, Ridge said. The project was finished five months ahead of schedule.

"We live in serious times and this is sobering information about those who want to do us harm," Ridge said. "Efforts each of you make to be vigilant ... do make a difference. Every citizen, using their common sense and their eyes and ears, can support our national effort to stop the terrorists."

In the summer months, "our minds tend to drift away from things like terrorism, but we have to remain vigilant," Brookes said. People need to know "if they see something suspicious, they need to report it."

Trying to Avoid Crying Wolf

Some lawmakers said the warning should be taken in cautious stride.

"This is not a major announcement, it's the reality of an increased risk in the homeland over the next several months," said Senate Majority Leader Bill Frist, R-Tenn. "There's, obviously, no reason for panic, or paralysis."

"The terror threat is certainly there, but you have to walk that fine line to make sure you don't cry wolf too many times and miss the chance to make the point," Sen. Kay Bailey Hutchison, R-Texas, told FOX News. "I think the terror threat is significant. I think it will be from now until the election ... we have to prepare ourselves."

California Rep. Jane Harman, the ranking Democrat on the House Permanent Select Intelligence Committee, said that what Ridge failed to address just how "broken" the current U.S. intelligence analysis and collection system is.

But "I think the warning is a serious warning — I think Al Qaeda is already here," Harman added. "However I think the color-coded system should be retired."

Rep. Edward Markey, D-Mass., recently sent a letter from Massachusetts and New York lawmakers to the House Appropriations Committee supporting a request by Boston and New York for $50 million to help defray convention security costs.

"Our letter cites several factors that have greatly and rapidly increased the security cost burdens on Boston and New York, including the bombings in Madrid and Moscow and recent warnings that Al Qaeda intends to 'hit the United States hard' in the next few months," Markey said.

"A successful attack at either convention would be a devastating blow to the political process and to our country's collective psyche."

Ridge said "there are constitutional and security questions" that are currently being pondered as to what action to take should an attack disrupt either convention.

Fox News' Catherine Herridge and Greg Kelly contributed to this report.