But what about the 15,000 facilities across the United States that produce or store deadly chemicals - chemicals that terrorists could use against us as weapons of mass destruction?
As CBS News Correspondent Steve Kroft first reported last November, the Justice Department calls that threat "real and credible."
Yet, almost three years after Sept. 11, chemical plants are still not subject to federal regulations when it comes to security.
Just a few miles across the Hudson River from New York City, tucked underneath a heavily-trafficked overpass, sits a non-descript chemical plant that manufactures disinfectant.
According to government records, nearly a thousand tons of deadly chlorine gas is stored here - the first agent ever used in chemical warfare during World War I.
"This is one of the main thoroughfares for commuters who come in and out of New York City every day," says Sen. Jon Corzine, D-N.J., who says there's very little security at this plant. "You know, looks to me like you could drive a truck through some of these fences if you wanted to pretty quickly."
So how many people could be affected here in the event of a terrorist attack? According to Corzine, 12 million people – the population living within a 14-mile radius of the plant that could be affected if a cloud of chlorine gas was released.
"We're looking all over Iraq for biological and chemical weapons. We don't have to look for 'em very hard, they're right here, right here in our backyards," says Corzine.
There are more than 100 chemical plants - in backyards all across the United States - where a catastrophic accident or an act of sabotage by terrorists could endanger more than a million people. One plant in Chicago could affect almost three million people. And in California, the chemicals at one site have the potential to kill, injure or displace more than eight million people.
If you're wondering who came up with these jaw-dropping statistics, they came from the chemical companies themselves. Federal law requires them to file a "risk management plan" with the Environmental Protection Agency, describing the "worst" case scenario that could happen at their plant.
"I think that one of the things that everybody has to understand about the business of chemistry is that we're in the risk management business," says Greg Lebedev, the president of the American Chemistry Council, which represents 150 of the largest chemical companies in America.
He contends that members are doing everything possible to ensure plant security, but do these reported worst-case scenarios alarm the council?
"I'm telling you that a 'worst case scenario' is a hypothetical example," says Lebedev. "It is merely used to illustrate the fact that we should secure chemical facilities. And the good news is we do."
But that's not what 60 Minutes found in visiting dozens of plants in major metropolitan areas that could put more than a million people at risk in the event of a terrorist attack. We found gates unlocked or wide open, dilapidated fences, and unprotected tanks filled with deadly chemicals that are used to manufacture everything from plastics to fertilizer.
At one facility in a suburb of Los Angeles, there was an impressive-looking front gate, but if there were security guards out back, we didn't see them. We did find a school, and a day care center less than a mile away.
And in the center of Houston, where a terrorist attack might affect three million people, it looked as if an intruder could simply walk right in.
The person who may know the most about the lack of security is Carl Prine, an investigative reporter at the Pittsburgh Tribune-Review, who began probing security at chemical plants six months after Sept. 11 -- after companies had been warned by the government that they were potential targets.
"I found almost non-existent security in a lot of places," says Prine, who visited 60 plants all over the country, including the Chicago, Baltimore, Pittsburgh, Pittsburgh and Houston area. "I walked right up to the tanks. There was one plant in Chicago, I simply sat on top of the tank and waved 'Hello, I'm on your tank.'"
And he did it in broad daylight, wearing his press badge and carrying a camera. He says no one tried to stop him.
"I began to wonder, I mean, what would it take for me to get arrested at one of these plants? Would I have to come in carrying an AK-47? What would it take for someone to say, 'Why is this guy walking around taking pictures of our tanks,'" says Prine, who told the chemical plants that he had been inside. "That was our policy. We took pictures so that the plants could identify their own weaknesses. And we also took our information to EPA; we took it to the American Chemistry Council."
But even after his expose ran in the newspaper, Prine was convinced that he could still get back into the same plants again. 60 Minutes asked him if we could tag along one rainy afternoon to see just how close they could get to the most dangerous chemicals at the Neville Chemical Plant outside downtown Pittsburgh.
There was an open gate right in front of the most dangerous chemicals at the plant. We made it in, with plenty of time to find what they were looking for.
"This is anhydrous ammonia. It searches out wet parts of the body. It goes right down your throat, rips out your lungs and also blinds you. It goes right for your eyeballs," says Prine. "If you were to blow this thing you would probably take out the plant."
But the most dangerous chemical at this plant was boron triflouride, a deadly, colorless gas with a suffocating odor that attacks mucous membranes and can kill at concentrations as low as 50 PPM.
"This is the one that kills Pittsburgh right here," says Prine. "It's a particular nasty gas."
By this time, we had been inside for more than 10 minutes, moving around at will. Prine suggested that we "see if anyone will stop us."
No one did. There wasn't one security person on site. All you had to do in most cases was wave, and others would smile back.
"Oh yeah, friendly wave," says Prine. "These are some of the friendliest employees I've found so far."
After awhile, we decided to leave, and it wasn't until after we were off the property that someone finally called out and asked what we were doing.
"What if we had been terrorists? That boron trifluoride would be up now, wouldn't it," says Prine.
The head of security asked us to walk with him. We agreed to go quietly to the front office, passing along the way a few more open gates. The management at the plant decided to call the police, who asked us to turn off the camera.
We were cited for trespassing, which carries a fine of $25, plus court costs.
When we told Lebedev that we had walked into the Neville Chemical Plant without a problem, he said, "That is an absolutely unacceptable security situation. And it underscores the point that you and I have been discussing - that we work every day on being better at security. One security breach in one facility or several facilities is unacceptable to us."
According to Prine, this is not an anomaly, but Lededev suggests that it is probably an exception to the rule: "There are 15,000 facilities in the United States, and the majority of them, I would argue, are safe and secure. But if one gate's left open or if you scale one wall, that's not acceptable."
Lebedev claims that Neville Chemical is no longer a member of their organization, even though it is still listed on their Web site as a member.
"We're gonna have to remove them from the website," says Lebedev.
The American Chemistry Council is requiring all its members to have "enhanced security" in place by the end of 2004. But only 7 percent of the 15,000 chemical facilities in the United States are members of that organization.
This is why Sen. Corzine introduced a bill - six weeks after Sept. 11 - putting the federal government in charge of chemical plant security. He said he expected quick passage, but he under-estimated the clout of the $450 billion dollar chemical industry.
"My bill was crushed by the American Chemistry Council. It was crushed by those who were looking after their private interests and not the public interests," says Corzine.
The American Chemistry Council did oppose Corzine's bill -- which required chemical companies to stockpile less chemicals on site and to use safer technologies whenever possible. But Lebedev now says they would welcome "partnership" with the federal government.
"We don't want you wandering into one chemical facility," says Lebedev. "And a piece of legislation is a good step to further that direction."
"I think government has to get into this game. And the sooner the better," says former senator Warren Rudman, who helped lead a blue ribbon task force on homeland security.
He warns that chemical plants are prime targets for terrorism: "I mean, it is an extraordinarily serious problem. You have to only look at the television footage from Bhopal in India when an accident occurred to recognize how serious a disaster this would be."
The accident in Bhopal in 1984 killed 3,800 people - more than those who died at the World Trade Center. Another 200,000 people suffered debilitating injuries, many permanent, when a cloud of methyl isocyanate was released from a Union Carbide pesticide plant.
"If you were terrorists and you decided to cause a major disaster, why would you not go to a plant that if you could penetrate it and blow part of it up, would cause fumes to waft over the entire area to kill who knows how many people," says Rudman.
What would he do with those plants?
"What I would recommend is that federal security experts working with the Congress establish minimum standards," says Rudman. "That Congress pass a law enabling the Department of Homeland Security to set those standards and to enforce those standards."
Tom Ridge, the head of Homeland Security, has said publicly that chemical plants shouldn't be allowed to regulate themselves, that voluntary efforts aren't enough. But that was a year ago and nothing has been done.
"You know, the threat is just staring us in the face. I mean, all you'd have to do is to have a major chemical facility in a major metropolitan area go up and there'd be hell to pay politically," says Rudman. "People will say, 'Well, didn't we know that this existed?' Of course, we knew."
About the only thing that has changed since 60 Minutes first aired this report is that, two weeks ago, Greg Lebedev resigned as president of the American Chemistry Council.
Legislation that would give the Department of Homeland Security the authority to set federal security standards at chemical plants has been languishing in the Senate for more than a year. That bill, however, does not give Homeland Security any criminal enforcement powers if a company refuses to comply.