As a custodian, he has access to the hazardous waste containment vault, and may have knowledge of hospital security practices and personnel. He once had ties to drug traffickers and to an African-American prison gang called the Black Gangster Family. The drug traffickers were low-level street dealers in the neighborhood where he grew up. He probably affiliated himself with the prison gang for personal protection.

You think a prison gang, or a street gang, was behind this?

She shook her head. Absolutely not. I mention Bryants affiliations because hes still a loose end. To access and erase a database, as well as hijack a video surveillance system, requires technical expertise not generally associated with street or prison gangs. Were thinking the level of sophistication and the materials stolen suggest a terrorist sleeper cell.

What can they do with the chemicals? Don said.

It has radiological dispersion device written all over it, Trudy said.

Dirty bomb, Luke said.

Bingo. Theres no other reason to steal radioactive waste. The hospital doesnt know the amounts that were taken, but they know what the stuff was. The chemicals include quantities of iridium-192, caesium-137, tritium, and fluorine. Iridium is highly radioactive, and concentrated exposure can cause burns and radiation sickness within minutes or hours. Experiments have shown that a tiny dose of caesium-137 will kill a forty-pound dog within three weeks. Fluorine is a caustic gas dangerous to soft tissue like the eyes, skin, and lungs. At very low concentrations, it makes eyes water. At very high concentrations, it inflicts massive lung damage, causing respiratory arrest and death within minutes.

Wonderful, Don said.

The important takeaway here, Trudy said, is high concentrations. If youre a terrorist, for this to work, you dont want a wide dispersal area. That would limit exposure. You want to pack a bomb with the radioactive material and a conventional explosive like dynamite, and you want to set it off in an enclosed space, preferably with a lot of people around. A crowded subway train or subway station at rush hour. Commuter hubs like Grand Central Terminal or Penn Station. A large bus terminal or airport. A tourist attraction like the Statue of Liberty. The enclosed space maximizes radiation concentrations.

Luke pictured the narrow, claustrophobic stairwell that climbed to the top of the Statue of Liberty. On any given day, it was mobbed with people, often school children on field trips. In his minds eye he saw Liberty Island filled with ten thousand tourists, the ferries jammed with even more people, like refugee boats from Haiti.

Then he saw the subway platforms of Grand Central Terminal at 7:30 a.m., so crowded with commuters that there was nowhere to stand. A hundred people would be lined up on the stairs, waiting for a train to come in and the platform to clear so the next group of people could descend. He pictured a bomb going off amongst that crowd.

And then the lights going out.