Blood Spatter and Newton's Third Law

Dr. Tom Harper thought he had committed the perfect crime. He had shot his wife in a manner that should have left the bullet hard to find and identify. Then he had repeatedly stabbed her dead body with a kitchen knife, and after wiping off any fingerprints, dropped the knife in their backyard. Nearby, he made several impressions in the soft dirt with a sneaker that was two sizes larger than his shoe size. The house had been ransacked, and jewelry and some other item were missing. It looked like the perfect setting for a random house invasion scenario.

Establishing an Alibi

Dr. Harper, the director of a large dental clinic, then left his house through the back woods and picked up his car, which was on an isolated road about a half mile away. One of the advantages of living way out in the country was in having no neighbors close by to see things they shouldn't. Harper proceeded to his office and went into a 9:00 am meeting with his staff. While he was committing this crime, he periodically went to his laptop and typed an e-mail to several people to establish that he was in his office all the time. To bolster his alibi, he sent the e-mails to his office computer using remote desktop software, and then forwarded them to the recipients. There would be no trace of the e-mails coming from his house.

Alice Harper was a large woman who worked out regularly at the gym. Tom Harper knew that he would never be able to stab her without her fighting back and leaving signs of a struggle. His plan was to shoot her in the back at close range with a small caliber .22 pistol. His knowledge of anatomy allowed him to shoot her in a way that would penetrate the abdominal aorta and cause her to immediately bleed out into her abdomen. Once on the floor, he would stab her repeatedly with a kitchen knife. Hopefully, at autopsy, they would only look at the stab wounds and not look for a small gunshot wound in her back. To ensure this, he also stabbed her in the back several times, including through the gunshot wound area.

The Problem

But this plan left a problem: the bullet. Surely the medical examiner would find the lead slug. He had looked into frangible .22 bullets, something like a CCI Quik-Shok, but they only fragmented into two or three pieces and the likelihood of one or more being found would be too great. This kind of bullet would also tear up some other organs, and he wanted this to be a clean shot into the aorta. When he stabbed her, he would be sure to hit the aorta first in the front, making it look like a knife wounds had ruptured it.

This problem plagued him until one day, as he was rebuilding a patient's broken tooth, he had a thought. What if he removed the bullet from the .22 shell and built up a bullet made of dental restorative composite, like Gradia or some other polymer material? If he made the material not too smooth on the surface, it would probably be overlooked as a bone fragment, and he could color the composite to match bone. On studying some literature, he found that by incorporating calcium phosphate gel into the composite when he made it, the bullet would be strong enough to penetrate the body, but then break into several pieces. Furthermore, if he stabbed her in the stomach, the hydrochloric stomach acid would leak out, and could dissolve the calcium gel, degrading the bullet fragment even further.

The Crime

Harper was convinced his scheme was foolproof. He prepared several bullets, then shot them into watermelons out in the woods, adjusting the polymers till he got the effect he wanted. He picked a Thursday morning to shoot her, knowing that her best friend would come at around 10:30 am to pick her up to go shopping. His last task was to break the window in the back door at the kitchen, the point of entry for the robber.

When her best friend called 911 reporting her dead, the local crime scene investigators and coroner soon arrived at the Harper residence. The coroner did a preliminary exam at the scene, and determined that the first knife wound had apparently penetrated the aorta. This would explain the lack of significant bleeding at the site of the other knife wounds.

Severing of the aorta caused her to bleed into the abdominal cavity so fast that her blood pressure fell, and blood was not readily supplied to the other wound areas. Tom, who was sitting nearby appearing to grieve over his wife's death, heard the coroner tell the investigators this. He mused to himself that they had bought it, so far.

The CSI team finished their work hours later, but found little physical or trace evidence to go on. The murder weapon was found on the lawn, but wiped clean of prints. They sent it to the lab to be superglue-fumed for latent prints. Cast of the large shoe prints in the soft dirt were made, and a variety of fibers and hairs were bagged, but they all would turn out to be either those of Alice, Tom, or their son Alec, who was away at college.

A Lingering Suspicion

Still, to investigators it looked a little too clean--a little too perfect. Knowing that the husband is always a potential suspect in homicides, Harper cooperated with the police in all interrogations. A co-worker who was interviewed mentioned that Harper had worn a red flannel shirt the day of the murder. On a hunch, the lead CSI asked Tom for the clothes he was wearing that day. Since Harper was right handed and also wore his watch on his right hand, the watch was also collected. Harper did not see any problem with giving them these items; after all, he wasn't there at the time of the crime.

Newton Gets His Man

Dr. Harper, a distinguished dentist in the community, forgot a basic principle of physics: to every action there is an equal and opposite reaction. Yes, Newton's Third Law of Motion was about to spoil the good doctor's plan. Analysis of the red shirt at the crime lab revealed a radiating pattern of small mist-like blood droplets within the red fabric of the shirt. The victim was shot with a small caliber .22, which would only produce blood spatter for a distance of two or three feet. To get his shot positioned right to hit the aorta, Harper had walked up to within a foot of his wife before he pulled the trigger. Similar analysis of the watchband with Luminol and an ultraviolet alternative light source showed bloodstain on the band. A sample of this blood submitted for DNA analysis established that the blood was that of Alice Harper.

Further Evidence

To solidify the case, they also found gunshot residue (GSR) on the shirt. The potential residue particles were lifted from the fabric with double sided tape, and then subjected to scanning electron microscope analysis. Now the victim's clothing and back were tested for GSR. GSR on Harper's shirt matched GSR removed from Alice Harper. One again, Newton's Third Law came into play in solving this crime.

The doctor's plan to commit the perfect crime was spoiled by a principle he learned in a high school physics class. But more important is that good forensic analysis provided the evidence necessary for another successful conviction.