"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 assault 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 cluster 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 space 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 o.f 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 analyzed. 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.

Chapter 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.

"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 severai 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 TM and crossed at a point headhigh 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?"

"This trail is one of five we were able to identify. Generally, assuming the blunt injury trauma is the only source of blood, or at least the first, the number of trails equates to the number of blows plus two."

"Why plus two?"

"There wouldn't have been any blood on the first blow On the second blow blood is picked up by the weapon and thrown off as the attacker makes the backswing for the third blow"

"Right."

"This medium-velocity spatter was found down low on the walls, and on the crap stacked in the corner.