Forensic Friday: Bloodstain Pattern Analysis
Blood is evidence you can’t talk your way out of, unless the wrong person interprets it. Bloodstain Pattern Analysis (BPA) is the science of studying the size, shape, and distribution of blood at a scene to reconstruct the events that caused it. Done right, it can reveal the type of weapon, the number of blows, the positions of people involved, and even whether a suspect’s story holds up. Done wrong, it can help send an innocent person to prison.
Who Interprets the Stains?
BPA isn’t just a skill you pick up from watching Dexter. Qualified analysts have formal training, often through programs recognized by bodies like the International Association for Identification (IAI) or the Organization of Scientific Area Committees for Forensic Science (OSAC).
The Innocence Project has highlighted that many wrongful convictions involved “experts” with minimal training, sometimes just a week-long course. The National Academy of Sciences (NAS) 2009 report on forensics criticized BPA specifically for lacking standardization and rigorous scientific validation, noting that conclusions are sometimes presented with more certainty than the science supports.
NAS Report: Strengthening Forensic Science in the United States
Innocence Project on BPA: Misapplication of Forensic Science
What Patterns Mean and What Causes Them
BPA breaks patterns into three main categories:
Passive Stains: Formed by gravity alone. Think drops from a bleeding nose, a hand dripping after being cut, or blood falling from a weapon held still. The shape depends on the surface: smooth tile makes round drops with crisp edges; carpet absorbs and distorts them.
Transfer Stains: Created when a bloody object touches a surface. This includes smears, wipes, or prints (like a bloody hand on a wall or a shoe print on the floor). They can show movement, direction, and even the specific object’s texture.
Projected Stains: Blood propelled by an external force. This is where things get more dramatic:
Impact spatter: Mist-like spray from a gunshot, or larger droplets from a blunt-force blow.
Cast-off: Arcs of blood flung from a weapon during repeated swings. Counting the arcs can help estimate the number of blows.
Expiration patterns: Blood forced from the mouth or nose due to internal injuries, sometimes mixed with saliva or mucus.
Each type has subcategories, and the interpretation depends heavily on scene context (one stain in isolation means little).
Case Examples: When It Works
In the Wayne Williams Atlanta child murders (1979–1981), BPA was part of a larger forensic puzzle. Analysts examined patterns in his home and car, matching them with fibers and other evidence to support the timeline of attacks. The bloodstain patterns aligned with the prosecution’s theory that the victims had been attacked and moved in specific ways, reinforcing the sequence of events.
Case Examples: When It Fails
David Camm, a former Indiana State Trooper, was accused of killing his wife and two children in 2000. The state’s key BPA witness claimed tiny spots on Camm’s T-shirt were “high-velocity impact spatter” from a shooting, placing him in the room when the shots were fired. Years later, independent experts re-examined the stains and concluded they were consistent with transfer, Camm had performed CPR on his son, and the patterns matched that action.
The wrongful convictions (two trials, both overturned) show how bias and lack of scientific rigor can warp BPA into a dangerous tool. The Innocence Project points to BPA misuse in several other cases, often where analysts overstated conclusions without proper controls or peer review.
Joe Bryan (Texas, 1985) A high school principal convicted of murdering his wife, Bryan’s case hinged on BPA testimony from a police officer with 40 hours of training. The analyst claimed specks on a flashlight proved Bryan shot his wife in bed. Later reviews by accredited BPA experts debunked the interpretation. Bryan was paroled in 2020 after 33 years.
Julie Rea (Illinois, 1997) Convicted of killing her 10-year-old son, Rea’s case included BPA testimony from a so-called expert later discredited in multiple trials. He claimed her account of a home invasion didn’t match the scene’s blood evidence. In reality, a serial killer confessed, and further review found the BPA conclusions unreliable. She was acquitted in 2006.
Why Standards Matter
The NAS report called for all pattern evidence disciplines, BPA included, to adopt uniform standards, undergo scientific validation, and include uncertainty estimates in testimony. OSAC now works to develop and publish best practices, but adoption isn’t universal. Without standardized methodology, one analyst’s “gunshot mist” could be another’s “transfer smear,” and juries rarely know the difference.
How Analysts Work the Scene
A methodical BPA process includes:
Documenting: Photographs at multiple distances, lighting conditions, and angles.
Mapping: Diagramming stain locations and sizes.
Classification: Sorting stains into passive, transfer, and projected types.
Trajectory analysis: Using strings, lasers, or software to track droplet paths back to a point of origin.
Correlation: Comparing patterns with other evidence, e.g. witness statements, autopsy results, weapon types.
A credible analyst will always include limitations in their report. They’ll say when evidence is inconclusive and avoid “absolute” language unless the science supports it.
The Takeaway
Bloodstain Pattern Analysis can be a powerful investigative tool, but only if applied under strict scientific controls. It’s not enough to know what the patterns could mean, the analysis must be objective, peer-reviewed, and grounded in tested methodology. Anything less, and you’re not reconstructing a crime. You’re telling a story you want to believe.
Your turn: Have you seen a case where BPA was used well or one that went disastrously? What convinced you?
