“My agent was a woman,” answered Seymour. “And she was there from the beginning.”

Managing the agent and her intelligence, Seymour continued, proved to be a delicate balancing act. Because the agent was highly placed within the organization, she often had advance knowledge of attacks, including the target, the time, and the size of the bomb.

“What were we to do?” asked Seymour. “Disrupt the attacks and put the agent at risk? Or allow the attacks to go forward and try to make sure no one gets killed in the process?”

“The latter,” replied Gabriel.

“Spoken like a true spy.”

“We’re not policemen, Graham.”

“Thank God for that.”

For the most part, said Seymour, the strategy worked. Several large car bombs were defused, and several others exploded with minimal casualties, though one virtually leveled the High Street of Portadown, a loyalist stronghold, in February 1998. Then, six months later, MI5’s spy reported the group was plotting a major attack. Something big, she warned. Something that would blow the Good Friday peace process to bits.

“What were we supposed to do?” asked Seymour.

Outside, the sky exploded with lightning. Seymour emptied his glass and told Gabriel the rest of it.

On the evening of August 13, 1998, a maroon Vauxhall Cavalier, registration number 91 DL 2554, vanished from a housing estate in Carrickmacross, in the Republic of Ireland. It was driven to an isolated farm along the border and fitted with a set of false Northern Ireland plates. Then Quinn fitted it with the bomb: five hundred pounds of fertilizer, a machine-tooled booster rod filled with high explosive, a detonator, a power source hidden in a plastic food container, an arming switch in the glove box. On the morning of Sunday, August 15, he drove the car across the border to Omagh and parked it outside the S.D. Kells department store on Lower Market Street.

“Obviously,” said Seymour, “Quinn didn’t deliver the bomb alone. There was another man in the Vauxhall, two more in a scout car, and another man who drove the getaway car. They communicated by cellular phone. And we were listening to every word.”

“The Security Service?”

“No,” replied Seymour. “Our ability to monitor phone calls didn’t extend beyond the borders of the United Kingdom. The Omagh plot originated in the Irish Republic, so we had to rely on GCHQ to do the eavesdropping for us.”