"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 wil 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 careful y at the shape of the stains.

Tick.

"Spatter size. Smal er or slower-moving forces produce large spatters, while larger or faster-moving forces produce smal er spatters.

He stopped, thumb pressed to his fourth finger

"Stil with me?"

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

"Give me examples."

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

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

"Beef blood," he explained.

I fol owed 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 smal chamber looked like a massacre site. Blood was pooled along one baseboard, streaked and splattered on the wal s, 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 al owed it to drop onto the paper under his feet.

"Low-velocity-impact spatters are associated with drops passively fal ing onto a surface. Dripping blood, for example. The characteristic spatter size is greater than three mil imeters in diameter. In these situations the blood is moving slowly, from normal gravitational pul to up to five feet per second."

I examined the smal 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 smal amount of blood into a dish, signaled me to step back, then swung the rod into it. The blood flew up and struck the wal . Gilbert gestured me over and pointed to several of the stains. They were smal er than the ones at his feet,

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

He lay down the rod.

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