Exactly how many other facilities exist nationwide is a mystery.
Ammonium nitrate is an important industrial fertilizer and mining explosive that, stored correctly, is stable and safe. But industrial history is dotted with dozens of deadly accidents involving the chemical.
Before Texas, the most recent incident occurred at a fertilizer factory in Toulouse, France, in 2001. An explosion killed 31, prompting France to pass a law requiring tougher regulations on the chemical.
Texas investigators still don't know what caused the fire that triggered the West explosion, but the devastation was a reminder of the chemical's power. Anti-government terrorist Timothy McVeigh used a truckload of ammonium nitrate to destroy the Alfred P. Murrah Federal Building in Oklahoma City in 1995.
Because of that explosive potential, if a fire were to break out at an ammonium nitrate company, everyone within a quarter- to a half-mile radius could be at risk, according to scientific papers. Debris from the Texas explosion landed more than two miles away.
In the states that provided verifiable data, the AP's analysis found more than 600,000 people who live within a quarter-mile of a facility, a potential blast zone if as little as 190 tons of ammonium nitrate is detonated. More send their children to school or have family in hospitals in those blast zones.
More often than not, census data show, the danger zones are middle-class or poor neighborhoods.
In the western Michigan farming town of Shelby, the Rev. Ruth D. Fitzgerald said she walks by the local branch of the Helena Chemical Co. every day. Her church is just around the corner.
The building doesn't look like a factory, she said, so she never thought about what was there. State records show that the company, which sells fertilizer to large farms, orchards and golf courses, reported storing as much as 1 million pounds of ammonium nitrate on any given day last year.
"I don't have any understanding of this at all," Fitzgerald said.
Recently, an abandoned house caught fire a half a block away from the chemical company, said Tim Horton, a real estate agent who sits on the local hospital board and the Shelby Area Chamber of Commerce.
Horton also didn't know how much ammonium nitrate was there: "I would say people don't know and don't care."
"Ignorance is bliss," he said.
And that's in a state where officials make the information available.
More than a half-dozen others, including Ohio, Connecticut, Hawaii, Idaho and South Carolina, refused to provide such information to the AP, citing the risk of terrorist attacks and their interpretations of federal law. Others, such as West Virginia, said the AP had to review paper records in person or request records one by one.
The result is a peculiarity of the post-9/11 age: Statistically, Americans are more likely to be hurt from chemical or industrial accidents like the one in Texas than from terrorist attacks like the one in Boston. Yet information intended to keep people safe is concealed in the name of keeping people safe.
Since the 1980s, states have been required under the Emergency Planning and Community Right-to-Know Act to tell people where dangerous chemicals are stored and how much is nearby.
That law followed a chemical leak in Bhopal, India, that killed more than 1,700 people and another in West Virginia that led to an evacuation. Ammonium nitrate has been responsible for some of the largest industrial disasters in history. In fact, what remains the worst industrial accident in the nation's history was an ammonium nitrate-triggered explosion in 1947 that killed more than 570 people in Texas City, Texas, and injured about 5,000.
But times have changed. Fears of chemical spills have given way to fears of terrorism.
In Hawaii, for example, officials said people must prove a "need to know" before they can obtain information. Though the state did not respond to a request for an explanation, the policy echoed others that cited a 2007 federal law intended to protect chemical plants from terrorist attacks. But the need-to-know requirement does not apply to the data submitted for Emergency Planning and Community Right-to-Know, said Bob Stephan, a former Homeland Security Department assistant secretary who was in charge of the U.S. government's chemical facility anti-terrorism program from 2007-09.
"They are giving you incorrect information or incorrect rationale for not providing the data," Stephan said.
Under Hawaii's interpretation of the law, people who want information about specific chemical facilities near their homes are qualified to see it. But that presupposes they already know enough to ask. Clarence Martin of the state's Hazard Evaluation and Emergency Response Office said people deserve to know what's in their neighborhoods.
But, he added, "I'm not going to let you tell them."
Even when the information is available, though, it's not always accurate. Years of lax oversight and scant enforcement have resulted in shoddy records. Hundreds of companies listed approximate or inaccurate amounts of dangerous chemicals, not just ammonium nitrate.
For instance, data from Louisiana said a Jimmy Sanders facility stored nearly 50 million pounds of ammonium nitrate. But the company said it never had any at all.
Others misidentified their locations. One plant in Tucson, Ariz., listed an ambiguous address ("end of cement plant road") and a geographic coordinate so off base that the Environmental Protection Agency's reporting software flagged the facility as being in a different county.
Arkansas reported that the Polk County Farmers Association stored 50,000 pounds of ammonium nitrate in the rural town of Mena. But the store's manager, Paul Stanley, said it had been moved to a facility about three miles outside of town years ago.
"I'm happy that it's not in town," he said, "because people don't understand it and they jump to conclusions."
Wisconsin documents showed that the C. Reiss Coal Co.'s facility had stored tons of ammonium nitrate in a facility in Sheboygan last year. But people would be hard pressed to use that information when deciding where to buy a home or send their kids to school. That's because state officials say the facility is inactive and should not have been on the list.
The fertilizer building that exploded in West had been there since 1962. As the years passed, a nursing home, school and apartment buildings sprung up nearby. Townspeople thought little of the facility; it was as common a sight in the farming community as a tractor driving down the road.
The company filed the required reports listing the hazardous chemicals on site. There's no indication that the documents were incorrect. But the county's emergency planners had not read them.
The Monroe County Co-Op in Aberdeen, Miss., stored as much as one million pounds last year, according to state records. But David Hodges, the store manager, said he had about half that on site and has sold it for about 50 years without a problem.
"I've been here, oh, 34, 35 years, and it's always been there," said Larry Middleton, a retired English teacher who lives up the street and visits to buy weed remover and snake repellent.
Horton said the same about the building in Shelby. Many townspeople have lived there all their lives, he said, and the fertilizer has been there, too. Though he didn't think most people knew the explosive potential, he said he feared that public knowledge of the building's contents would attract terrorists.
"I can't predict when an accident is going to happen. It just happens," he said. "Terrorists are actively seeking ways to harm us."
Behavioral scientists call this "probability neglect": People are far more likely to overreact to emotional, extremely unlikely events such as terrorism than to address potential problems that are far more likely to occur.
What's more, people are more afraid of risks brought on by outsiders, like terrorists, than threats closer to home. In experiments, people were more outraged by the thought of being exposed to radiation from nuclear waste than from radon in their own basements - even when they were told the danger was the same and the likelihood of radon exposure was much higher.
"It's been here all this time," Middleton said, "and nothing has happened."