Deadly Decisions_ A Novel Part 23

"Only when the 'pulling' forces exceed the 'pushing' forces will the drop break free. Initially it's elongated, but as it falls the drop flattens due to air resistance. The attractive forces of surface tension within the drop cause it to a.s.sume a shape with the least amount of surface area. Thus, drops of blood are shaped like spheres, not like teardrops as they're usually drawn. And shape is one of the things we consider in spatter-pattern a.n.a.lysis.

"A blood spatter is produced as a result of a force striking static blood. It could be in a pool on the sidewalk, or inside a victim's head. When hit, the blood breaks into drops, called spatter, which travel through the air as spheres."

I nodded.

"When these spheres strike a surface they leave predictable types of trails. Bloodstain-pattern interpretation is concerned with examining stains produced by drops of blood that are not not typical. The stains and trails have been altered in some way, usually by violent activity. typical. The stains and trails have been altered in some way, usually by violent activity.

"The goal of bloodstain-pattern interpretation is to work backward from a crime scene and reconstruct the events that took place. What happened? In what sequence? Who was where? What weapon was used? What objects have been moved? To answer these questions we look at what has altered the drops of blood present.

"And it's very complex." He began ticking points off on his fingers. "For example, we have to take into account the properties of the target. Blood will act differently when striking a smooth versus a textured surface."

Tick.

"Shape. Since the ratio of a stain's width and length accurately reflects its angle of impact, regardless of striking surface, we look carefully at the shape of the stains."

Tick.

"Spatter size. Smaller or slower-moving forces produce large spatters, while larger or faster-moving forces produce smaller spatters."

He stopped, thumb pressed to his fourth finger.

"Still with me?"

"Yep."

"We talk about low-, medium-, and high-velocity-impact spattering, although these terms are really relative."

"Give me examples."

"I'll do better than that. Come with me."

He led me down the hall to a stainless steel refrigerator and withdrew a one-liter bottle labeled Sang du boeuf Sang du boeuf.

"Beef blood," he explained.

I followed him along a narrow side corridor to an unmarked door, and we entered a windowless room where large sheets of white paper were taped to most surfaces.

The small chamber looked like a ma.s.sacre site. Blood was pooled along one baseboard, streaked and splattered on the walls, and dripping from various-sized spots at knee level in the far corner. Above each stain I could see pencil notations.

"This is our blood-spatter experiment room," said Gilbert, placing the bottle on the floor. "Watch."

He removed the cap, dipped a wooden rod into the blood, then allowed it to drop onto the paper under his feet.

"Low-velocity-impact spatters are a.s.sociated with drops pa.s.sively falling onto a surface. Dripping blood, for example. The characteristic spatter size is greater than three millimeters in diameter. In these situations the blood is moving slowly, from normal gravitational pull to up to five feet per second."

I examined the small round stains he'd created.

"Medium-velocity blood spatter results from activities such as beatings, blunt trauma injuries, or stabbings. The blood is moving faster, with a force velocity between five feet and twenty-five feet per second."

As he said this he poured a small amount of blood into a dish, signaled me to step back, then swung the rod into it. The blood flew up and struck the wall. Gilbert gestured me over and pointed to several of the stains. They were smaller than the ones at his feet.

"See these spatters? The size range for medium-velocity spatter is typically less, averaging between one and four millimeters in diameter."

He lay down the rod.

"But the spots aren't as fine as with high-impact spatters. Come look at this."

We moved to the far wall, where he indicated an area that looked as if it had been spray-painted.

"High-velocity-impact spattering means a force velocity greater than one hundred feet per second and results from gunshots, explosions, and mechanical accidents. It's more like a mist, with individual spatters averaging less than one millimeter in diameter.

"But don't get me wrong. Not every spatter falls neatly into one of these categories. Blood that is splashed, cast off, or projected can really complicate a picture."

"How so?"

"These are actually forms of low- to medium-velocity spatter, but they differ from the ones I just described. For example, splashed blood results from someone stepping into already-pooled blood. This leaves long, narrow spatters surrounding a central stain, with very few round stains present.

"Projected blood results from someone running through, stamping into, or slapping a pool of blood. Or from arterial gushing, or beating a head on the floor. Again there are long, spiny spatters radiating from a central stain. But in this case the borders of the central stain are also distorted.

"Blood cast off from a weapon leaves yet another pattern. Let me show you."

He went back to the rod, dipped, and swung it in an arc. Blood flew from the tip and struck the wall to his right. I drew close and studied the stain.

"Cast-off drops are smaller than those in typical low-velocity spatter, and the greater the force the smaller the drops. Also, since the blood is being thrown from a moving object, cast-off spatter occurs in straight or slightly curved trails, and the drops are fairly uniform throughout."

"So you can determine the nature of an a.s.sault based on the size and shape of the spatter?"

"Yes. And in most cases we can pinpoint where the attack took place. Let's go back to my office and I'll show you something else."

When we were once again in front of the computer he placed his hands on the keyboard and entered a command.

"You saw us taking videos of the bloodstains in the victim's apartment, right?"

"Yes."

"We used a simple video camera, but you can also use digital. We recorded each area of spatter using a scale and a plumb line."

"Why a plumb line?"

"The program uses that to determine the vertical direction of the stain."

Gilbert hit a key and a cl.u.s.ter of elliptical brown forms came up on the monitor.

"The images on the videotape are entered into the computer and can be played back on the monitor. Single frames are grabbed and recorded on the hard disk as bitmaps. A program then displays the image of each stain so we can take measurements. The measurements are used to calculate two angles: the angle of directionality, and the angle of impact."

More keystrokes, and an oval-shaped white outline appeared superimposed over the stain at center screen. Gilbert pointed at it.

"The direction of the main axis of the ellipse with respect to the plumb line defines the directionality angle, or gamma of a stain. That can range from zero to three-sixty.

"The impact angle, or alpha, can range from zero to ninety degrees. That's calculated from the shape of the ellipse."

"Why is that?"

"Remember, when a drop of blood travels through s.p.a.ce it's spherical. But when it strikes a target it flattens and leaves a trail. That's because the bottom of the drop is actually wiping across the surface."

He made a swiping gesture with his hand.

"The trail is small at first as the drop strikes, then widens, with the widest point of the trail corresponding to the center, or widest part of the drop. The trail then narrows and eventually tapers off. See this one here?"

He pointed to an elongated oval with a small dot at one end. It looked like many I'd seen in the blood-spatter room.

"It looks like an exclamation point."

"That's exactly what it's called. Sometimes a small dot of blood detaches from the original drop and hops to the head of the trail. So looking at a spatter from above, it resembles either a tadpole or an exclamation point, depending on whether the far end is merely elongated, or a small portion detached completely. In either case the direction of travel is clear."

"The dot points toward the direction the drop was moving."

"Exactly. The program produces a file containing the values of the angles for each stain a.n.a.lyzed. It's from that data that point of origin is calculated. And believe me, using the computer is a lot quicker than the old string method."

"Back up."

"Sorry. With the string method one end of a string is fastened to the surface at the position of the stain, then stretched in the estimated direction of motion. This is repeated for a number of bloodstains around the scene. The result is a pattern of strings extending away from the spatter toward the source of the blood. Home plate is the point where all the strings converge. The procedure is time consuming and leaves a lot of room for error. Instead of doing it by hand the computer draws virtual strings computed from the data."

His fingers flew over the keys and a new image appeared. X and Y coordinates ran down the left side and across the bottom of the screen. A dozen lines formed an X-shaped pattern, crossing each other in a geometric bow.

"This is a bird's-eye view of a set of virtual strings based on twelve spatters. It's hard to get this point of view with real strings, yet it's the most useful one."

More key clicking and a new image appeared. The lines now plunged together from upper left to lower right, converging at a point two thirds of the distance from the bottom of the screen, then spreading slightly, like stems on a bunch of dried flowers.

"The program can also produce a side view, which is necessary to estimate the height of the source of blood. By combining the two views you have a pretty accurate idea of point of convergence and, therefore, of victim position."

Gilbert leaned back and looked at me.

"So what do you want to know about the Cherokee scene?"

"Anything you can tell me."

For the next forty minutes I listened and watched, interrupting only for clarification. Gilbert was patient and thorough as he walked me through the bloodbath in the apartment.

What he said increased my conviction that Claudel was leading us in a dangerously wrong direction.

23.

THE SCREEN WAS FILLED WITH HUNDREDS OF TINY DOTS, LIKE the spray-paint mist in Gilbert's test room. Scattered among them were small bits of flesh and bone. the spray-paint mist in Gilbert's test room. Scattered among them were small bits of flesh and bone.

"You're looking at a section of the north wall, right behind the victim's chair. That's forward spatter."

"Forward spatter?"

"From the pellets exiting Cherokee's head. Blood from an entrance wound is called back spatter. Look at this."

Gilbert hit the keys and a new image filled the screen. It was a similar spray of aerosolized blood, though less densely packed, and lacking the larger globs of tissue.

"That's from the TV. When the pellets struck Cherokee, blood flew backward."

"He was shot sitting in the chair?"

"Yes."

He entered several more keystrokes and the image was replaced by a view of the chair where the body had been found. Lines ran diagonally from the wall and the TV, and crossed at a point head-high above the seat.

"But the gunshot was icing on the cake. If he wasn't dead already, he was well on his way. Look at this."

More keystrokes. Another image, this one with larger spots and more variation in their size.

"That's medium-velocity spatter. It was all over the northwest corner of the apartment."

"But-"

"Just wait."

He brought up another frame. This one showed spots slightly larger than those in the previous image, but of roughly uniform size. They varied in shape from round to ovoid.

When Gilbert hit a key and zoomed out I could see that most of this spatter was distributed in a long curving line, with some drops lying to either side of the arc.

"That's from the ceiling."

"The ceiling?"

"It's what we call a cast-off pattern. It results from blood being thrown from a moving object, like my stick. When swinging a weapon, the attacker terminates his backstroke abruptly, then reverses direction to deliver the next blow. Most blood flies off on the backstroke, at least if there's enough force, but some can be thrown on the downward stroke, too."

He pointed to drops in the center of the trail.

"These spatters are due to the backswing."

He indicated several drops lying along the edge of the arc.

"And these are downswing trails."

I took a moment to digest that.

"So you're saying that he was beaten before he was shot?"