You are working a murder case where a suburban housewife is found brutally murdered in her living room. The husband comes home and finds her at 5:40 in the afternoon. The coroner established time of death at about 10:00 am that morning. The husband has a rock solid alibi. He was on local TV station doing a live broadcast; thousands of people saw him.
There is no evidence that the wife or husband were having any type of extra-martial affair that might led to a jealous lover attacking her, There was no sign of a robbery, and the husband says that nothing is missing from the house. No sign of a struggle except a broken blue flower vase that is about three feet from the body. The woman appears to have been hit with the vase at least twice, resulting in a large hematoma in the brain. This has been ruled the cause of death. The victim's blood and hair are found on the vase. The only latent fingerprints on the vase are from the victim and their cleaning lady, who also has a solid alibi. The victim has a set of bruise marks on her right arm that look like she had been grabbed by a large hand, probably male. There is no DNA under her fingernails or any other evidence that might lead to DNA information on the perpetrator.
The family is not in financial trouble; no big life insurance policy on the wife, and all the neighbors tell of what a "happy couple" they were. So where does that all leave you? It's possible it was a random event, but there is no evidence of rape or robbery. It's possible that the husband hired someone to kill her, but what is the motive? No money incentive, no known young floozy on the side, just an apparently happily married couple. No one in the neighborhood appears to be a disgruntled neighbor. You are left with no apparent suspects. Where do you go from here?
The crime scene forensic team has combed the area for evidence with little outcome. They have collected, packaged and removed the broken flower vase, as well as the broken glass fragments from it, and processed them as evidence. The glass fragments are all quite small and scattered around the area where the victim was found. Many fragments were found in her hair and on her clothes. The vase was a fine crystal and the force of the killer's blows shattered it into many pieces. The few larger fragments were checked for latent prints by fuming with cyanoacrylate, but no usable partial prints could be recovered.
The investigators working the neighborhood have come up pretty much dry in terms of any leads as to what happened or who the perpetrator might be. Except for one thing--one neighbor on her way to the market around 9:00 am noticed a car parked at the corner nears the victim's house. She said there was a man in the car and he appeared to writing something on a pad. She thought he was the electric or water company's meter reader. She did notice that he was driving an "older car," maybe something from the 80s, maybe a Ford, but she was not sure.
On a hunch, one of the forensic technicians went to the corner looking to see if any tire or shoe tracks were left behind. What he found instead was broken glass--glass from a car headlight lens. At the corner was a large granite stone marker about 4 feet tall. It was the type towns sometimes use for surveying markers. It looked as if a car had stuck it, and since the marker was back from the road, it was safe to assume that the car may have been parked by the marker. While there was no way to tell when the car might have hit the marker or even if it had any relation to this crime scene, the CSI collected and tagged the broken glass for processing.
Back at the forensic laboratory
The trace technicians set about to determine the properties of the glass from the corner scene. A variety of analytical techniques were applied to the samples to identify any chemicals present that might have been added as stabilizers and fluxes. Analysis indicated that the glass was a borosilicate glass similar to that often used in car sealed-beam headlights. Sample density and refractive index (RI) measurement were also consistent with this view.
Sample material subjected to inductively-coupled plasma mass spectroscopy (ICP-MS) analysis and tests for chemical coatings on the glass surface yielded a specific profile for the glass. This was compared to the glass databases available from the FBI and car manufacturers, and indicated a high probability that the glass was from a 1980-1985 Ford.
A close friend of the victim indicated to investigators that the victim had told her she had received some unwanted calls from an old college boyfriend lately. With his name in hand, investigators determined that he had several cars, including a 1984 Ford Mustang SVO. Looking at the car in the parking lot where the suspect worked led investigators to believe that the right headlamp had recently been replaced.
Back at the trace lab again
The trace technician had completed analysis on the glass shards from the flower vase that was the murder weapon. The glass had a high lead content, and also an unusually high zinc oxide (ZnO) and strontium oxide (SrO) content. That and its fracture pattern, as well as color, density and RI allowed the lab to develop a profile for the glass.
The college boyfriend was brought in for questioning and was very uncooperative. He could not account for his whereabouts at the time of the crime, and admitted that he had several phone contacts with the victim recently. A search of his car and residence yielded glass shards similar to those from the purple flower vase from the driver-side carpet of his car. In addition, similar shards were removed from the soles of a pair of sneakers found in his closet. Further analysis of these sources also yielded traces of the victim's blood from blood spatter on the shoes and transfer blood spatter on the car seat.
At trial, the two pieces of glass trace evidence were critical parts of the prosecution's case. The members of the jury viewed these as strong circumstantial evidence.