Doyle had been clutching an evidence bag between his arm and torso. He removed it, studied the back side of the three photographs they’d taken from the hiding spot under Ricky’s couch, looked at his phone once more, then said, “If I’m understanding this correctly, one of the papier-mâché masks was taken on film manufactured in September of 1991. The second, as well as the plaster mask, were both manufactured in March of 1992.”

“That doesn’t mean the photograph was taken then.”

“Only that the film was manufactured and for sale,” Doyle confirmed.

Larkin took the evidence bag and studied the photographs with the professional detachment that made his job possible. “How long does Polaroid film last.”

“Hmm… it’s best to use most films within twelve months.”

“Well before Andrew’s murder, then,” Larkin murmured. He drew the pad of his thumb back and forth across one of the papier-mâché photos.

“What’re you thinking?”

Larkin said, “Children love penguins. Every year, Noah has a penguin week in class. They take a field trip to Central Park Zoo, watch some kid-friendly documentary, he reads them And Tango Makes Three, and they make papier-mâché penguins. The week before, I’m usually up all night helping him fill a trash bag with strips of newspaper. You have to prepare certain steps, of course—otherwise six- and seven-year-olds will never finish the project. But it’s a fairly simple craft.”

Doyle had pocketed his phone, crossed one arm over his middle, and seemingly unaware of it, was chewing on his thumbnail.

Larkin suppressed a smile at the return of Doyle’s fidgeting. “Plaster of Paris is beyond first graders.”

“It’s a bit more nuanced,” Doyle agreed. He met Larkin’s eyes. “The killer was experimenting with their signature?”

“It reads to me as a clear progression in artistic knowledge.”

Doyle hastily removed the notepad from his suit coat. He flipped to a blank page, turned it sideways, then drew a line across the length of the paper. “Papier-mâché in ’91. Again in ’92, followed by plaster of Paris. And then there’s Andrew with cast iron in ’98.”

“Clear escalation in both the perpetrator’s desire to commit murder, and the skill surrounding his ritual with the body.”

Doyle scribbled notes underneath the basic timeline. “Our questions would be: why does Ricky have these photographs, which are clearly souvenirs. Because you don’t think—”

“Ricky is the Bonnie to the unknown perpetrator’s Clyde,” Larkin said with finality.

“Who is Clyde, then?”

Larkin finally smiled. “Arguably our most important question.”

“Are there more than three victims?”

“Yes,” Larkin agreed.

“Why three women and then a man?”