The next attack: terrorists in Iraq are becoming proficient at blowing up oil refineries. Similar plants in a handful of American cities represent our greatest vulnerability. We could easily be making them less dangerous. But we're not.

• Author: Flynn, Stephen

Consider this. It's a warm Friday evening in June, and 40,000 baseball fans are gathered at Philadelphia's Cities Bank Park to watch the Phillies play the New York Mets. Most of the stadium's 21,000 parking spaces are filled. Just a few hundred yards away, Interstate 95 is crowded with weekend travelers. Two miles due west, workers on the night shift are starting to arrive at the thousand-acre Sunoco oil refinery on the banks of the Schuylkill River. A light breeze is blowing toward the east.

Across the Delaware River, three young men gather in a vacant lot. The leader of the group is a British national, a second-generation Pakistani from Liverpool who spent much of 2004 in Iraq. He has received training in bomb making from an Iranian tutor in southern Iraq and participated in attacks on Iraqi oil refineries. In the spring of 2005, a few months after returning from Iraq, the British-born jihadist traveled to the United States on a flight from London. Having never run afoul of the law in the United Kingdom, his name was not on the traveler watch list of the Transportation Security Administration (TSA). Further, as a British subject he was able to travel to New York without a visa.

With a letter of introduction from a radical British sheikh, the jihadist found his way to a Jersey City mosque. There, the local imam put him in contact with two Americans: a college student and his older cousin, both of whom had become adherents of the teachings of the radical Egyptian philosopher Sayyid Quth. The three hatched a plan to use a commercial tanker truck to target the Sunoco refinery in southern Philadelphia.

Thousands of tanker trucks operate in the tristate area of New York, New Jersey, and Connecticut. The older cousin holds a commercial driver's license and has been a truck driver for more than five years. In the summer of 2006, he provided biographical information and fingerprints to the TSA to get a hazardous materials endorsement added to his license. This allowed him to get a job driving for an independent gasoline distributor. The younger cousin obtained a job as an apprentice painter among the thousands of contractors who work on the Sunoco facility each day. His mission was to scout out the best target and the truck route to get to it.

In the late afternoon before the Phillies-Mets game, the older cousin drives his truck to the Valero Energy terminal and receives a full load of premium gasoline. Instead of making his first scheduled stop, he drives to a nearby rendezvous point to join up with his two coconspirators. The younger cousin is driving a pickup truck armed with a small fertilizer bomb. The British jihadist opens his car trunk and carefully removes two suicide vests packed with explosives. On the evening before, the two cousins had dressed in white and recorded their martyrdom videos.

Shortly after 7 p.m., as the fans at the ballpark rise to sing the national anthem, the tanker truck and pickup approach the entrance to the Sunoco facility. Abruptly, the driver of the pickup accelerates into the guard shack, detonating the explosives. The explosion knocks out the roll-away gate, allowing the larger truck to drive in. Soon, the British jihadist locates his target, a large storage tank that is well marked with hazard placards. Crying "Allah is great!" he detonates his vest. The explosion sends up a ball of flame 200 feet high, immediately killing everyone within a hundred yards of the truck. The secondary explosions kill many more employees, crippling the refinery's ability to put its emergency response plan in place.

Two miles away, fans at Citizens Bank Park go quiet when the concussive force and noise of the explosion reach them. The umpire stops the game as security officials scramble to get more information. At the refinery, the secondary explosions have ruptured pipelines to several smaller pressurized tanks that contain thousands of gallons of anhydrous hydrogen fluoride. The warm evening and the heat of the raging fires nearby cause the hydrofluoric acid to evaporate and form a concentrated, colorless vapor, which, carried by the light westerly wind, moves slowly across the interstate and toward the blue-collar neighborhood north of Roosevelt Park.

An announcement comes over the stadium's loudspeaker, saying there has been an accident at a nearby refinery. The crowd is directed to evacuate calmly and drive north, toward the center of Philadelphia. As families scramble to get to...