IN THE HISTOLOGY LAB I ASKED DENIS TO PULL OUT CASES 25906-93 and 26704-94. I cleared the table to the right of the operating scope and placed my clipboard and pen. I took out two tubes of vinyl polysiloxane and positioned them, along with a small spatula, a tablet of coated papers, and a digital caliper accurate to .0001 inch.

Denis placed two cardboard boxes on the end of the table, one large and one small, each sealed and carefully labeled. I eased the lid from the larger box, selected portions of Isabelle Gagnon?s skeleton, and laid them out on the right half of the table.

Next I opened the smaller box. Though Chantale Trottier?s body had been returned to her family for burial, segments of bone had been retained as evidence, a standard procedure in homicide cases involving skeletal injury or mutilation.

I removed sixteen Ziploc bags and put them on the left side of the table. Each was marked as to body part and side. Right wrist. Left wrist. Right knee. Left knee. Cervical vertebrae. Thoracic and lumbar vertebrae. I emptied each bag and arranged the contents in anatomical order. The two segments of femur went next to their corresponding portions of tibia and fibula to form the knee joints. Each wrist was represented by six inches of radius and ulna. The ends of the bones sawed at autopsy were clearly notched. I would not confuse these cuts with those made by the killer.

I pulled the mixing pad toward me, opened one of the tubes, and squeezed a bright blue ribbon of dental impression material onto the top sheet. Next to it I squirted a white ribbon from the second tube. Selecting one of Trottier?s arm bones, I placed it in front of me and picked up the spatula. Working quickly, I mixed the blue catalyst and the white base, kneading and scraping the two squiggles into a homogenous goo. I scraped the compound into a plastic syringe, then squeezed it out like cake decoration, carefully covering the joint surface.

I laid the first bone down, cleaned the spatula and syringe, tore off the used sheet, and began the process anew with another bone. As each mold hardened I removed it, marked it as to case number, anatomical site, side, and date, and placed it next to the bone on which it had been formed. I repeated the procedure until a rubbery blue mold sat next to each of the bones in front of me. It took over two hours.

Next I turned to the microscope. I set the magnification and adjusted the fiber-optic light to angle across the viewing plate. Starting with Isabelle Gagnon?s right femur I began a meticulous examination of each of the small nicks and scratches I had just cast.

The cut marks seemed to be of two types. Each arm bone had a series of trench-like troughs lying parallel to its joint surfaces. The walls of the troughs were straight and dropped to meet their floors at ninety-degree angles. Most of the trench-like cuts were less than a quarter of an inch in length and averaged five hundredths of an inch across. The leg bones were circled by similar grooves.

Other marks were V-shaped, narrower, and lacked the squared-off walls and floors of the trench-like grooves. The V-shaped cuts lay parallel to the trenches on the ends of the long bones, but were unaccompanied in the hip sockets and on the vertebrae.

I diagrammed the position of each mark, and recorded its length, width, and, in the case of the trenches, depth. Next I observed each trench and its corresponding mold from above and in cross-section. The molds allowed me to see minute features not readily apparent when viewing the trenches directly. Tiny bumps, grooves, and scratches marking the walls and floors appeared as three-dimensional negatives. It was like viewing a relief map, the islands, terraces, and synclines of each trench replicated in bright blue plastic.

The limbs had been separated at the joints, leaving the long bones intact. With one exception. The bones of the lower arms had been severed just above the wrists. Turning to the bisected ends of the radius and ulna, I noted the presence and position of breakaway spurs, and analyzed the cross-sectional surface of each cut. When I?d finished with Gagnon, I repeated the whole process for Trottier.

At some point Denis asked if he could lock something up, and I agreed, paying no attention to his question. I didn?t notice the lab grow quiet.

?What are you still doing here??

I almost dropped the vertebra I was removing from the microscope.

?Jesus Christ, Ryan! Don?t do that!?

?Don?t go bughouse, I just saw the light and thought I?d drop in to see if Denis was putting in overtime slicing up something entertaining.?

?What time is it?? I gathered the other cervical vertebrae and placed them in their bag.

Andrew Ryan looked at his watch. ?Five-forty.? He watched me lift the bags into the smaller cardboard box and set the cover on top.

?Find anything useful??

?Yup.?

I tapped the cover into place and picked up Isabelle Gagnon?s pelvic bones.

?Claudel doesn?t put much stock in this cut-mark business.?

It was precisely the wrong thing to say. I put the pelvic bones in the larger box.

?He thinks a saw?s a saw.?

I laid the two scapulae in the box and reached for the arm bones.

?What do you think??

?Shit, I don?t know.?

?You are of the carpentry and grout gender. What do you know about saws?? I continued laying bones in the box.

?They cut things.?

?Good. What things??

?Wood. Shrubbery. Metal.? He paused. ?Bone.?

?How??

?How??

?How.?

He thought a minute. ?With teeth. The teeth go back and forth and cut through the material.?

?What about radial saws??

?Oh well, they go around.?

?Do they slice through the material or chisel through it?

?What do you mean??

?Are the teeth sharp on the edge or flat? Do they cut the material or rip their way through it??

?Oh.?

?And do they cut when they go back or when they go forth??

?What do you mean??

?You said the teeth go back and forth. Do they cut on the back or on the forth? On the push stroke or on the pull stroke??

?Oh.?

?Are they designed to cut on the grain or across the grain??

?Does that matter??

?How far apart are the teeth? Are they evenly spaced? How many are on the blade? What?s their shape? How are they angled front to back? Is their edge pointed or squared off? How are they set relative to the plane of the blade? What kind of . . .?

?Okay, okay, I see. So, tell me about saws.?

As I spoke, I placed the last of Isabelle Gagnon?s bones in the box and tapped on the cover.

?There must be hundreds of different kinds of saws. Crosscut saws. Ripsaws. Pruning saws. Hacksaws. Keyhole saws. Kitchen and meat saws. Ryoba saws. Gigli and rod saws. Bone and metacarpal saws. And those are just the hand-powered ones. Some run on muscle, and some are powered by electricity or gas. Some move with a reciprocating action, some use continuous action, some move back and forth, some use a rotating blade. Saws are designed to cut different types of materials and to do different things as they cut. Even if we just stick to handsaws, which is what we?ve got here, they vary as to blade dimensions, and the size, spacing, and set of their teeth.?

I looked to see if he was still with me. He was, eyes as blue as the flame in a gas burner.

?What all this means is that saws leave characteristic marks in materials such as bone. The troughs they leave are of different widths and contain certain patterns in their walls and floors.?

?So if you?ve got a bone you can tell the specific saw that cut it??

?No. But you can determine the most likely class of saw that made the cuts.?

He digested that. ?How do you know this is a handsaw??

?Power saws don?t depend on muscle, so they tend to leave more consistent cuts. The scratches in the cuts, the striae, are more evenly patterned. The direction of the cut is also more uniform; you don?t see a lot of directional changes like you do with a handsaw.? I thought for a minute. ?Since there isn?t a lot of human energy required, people using power saws often leave a lot of false starts. And deeper false starts. Also, because the saw is heavier, or sometimes because the person working it is putting pressure on the object being cut, power saws tend to leave larger breakaway spurs when the bone finally gives.?