(CNN) -- George Zimmerman's defense team finished presenting its case in court this week by calling witnesses to testify about the physical evidence as it relates to Zimmerman's story. The physical evidence in this case has caused some roadblocks for both the prosecution and the defense, and, in fact, the way the evidence was collected and handled at the scene of the shooting could have impaired some critical information surrounding what happened that night.
Zimmerman is charged with second-degree murder for killing Trayvon Martin on February 26, 2012. Zimmerman says he shot the teenager in self-defense. Since the incident, there hasn't been one clear, detailed explanation of exactly what led to the altercation between Martin and Zimmerman, and how the altercation actually unfolded. The basic story has remained consistent, but the details of what happened could provide some critical insight into the story and regarding Zimmerman's self-defense claim.
Zimmerman's account of what happened that night has become a central part of this murder trial, not only because he's the only living person who witnessed the entire incident, but also because there really isn't much physical evidence for either team to fall back on.
Second-degree murder is defined as a killing that's carried out with hatred, ill will or spite, but is not pre-meditated. Knowing how a person is feeling during a killing is difficult to prove, therefore the prosecution has to rely on the evidence to tell the story of what happened and what led to the shooting, according to HLN's Ryan Smith. So without an abundance of evidence, proving this case becomes more difficult for both sides.
The lack of injuries found on Martin's body -- as reported in the official autopsy report -- is a major point that has raised some questions about the validity of Zimmerman's story. The shortage of evidence even led the defense to have an animation created that re-enacts how the altercation and shooting may have played out, based on available evidence, Zimmerman's story and witness accounts.
So why is there a shortage of evidence? Were shortcuts taken or mistakes made during the initial investigation of Martin's death and of the crime scene in general? Due to the gaps in the physical evidence, many experts seem to think so.
In order to get a better idea of what may have gone wrong that night, we asked HLN law enforcement analyst Mike Brooks and Joseph Scott Morgan -- who has worked in death investigation for 26 years and served on a 12-member team that set up death investigation standards for the U.S. Department of Justice -- to break down some of the key points they think police and investigators missed that night. Overall, Brooks and Morgan say how the evidence was collected at the scene and how Trayvon Martin's body was handled by investigators that night both could have caused some big problems.
A variety of adverse conditions played into the initial investigation that night. A dark, rainy scene isn't ideal for a death investigation; however, experts point out that the attitude of investigators could have also played a part in how things were handled. After Zimmerman referenced a series of robberies in the area to explain what led to his suspicion of Martin, did local police think they were dealing with the death of burglar? Also, how often does a small police department deal with a potential homicide scene? Law enforcement analyst Brooks says simply the lack of experience could have impaired this investigation.
Although the rain that night may have been unavoidable, Morgan says it shouldn't have caused as much damage as it did. According to Department of Justice standards, "The preservation and documentation of the evidence on the body must be initiated by the investigator at the scene to prevent alterations or contamination."
The standards for preserving evidence on a body say Martin's hands should have been secured in unused paper bags in order to preserve any trace evidence, including blood, fingerprints and anything else present at the scene. According to the official reports and photos, this was not done.
Dr. Vincent Di Maio, a former medical examiner in San Antonio, Texas, and a world-renowned expert on gunshot wounds, took the stand Tuesday as an expert for the defense. Defense attorney Don West asked Di Maio about the injuries Zimmerman suffered -- a broken nose in particular -- and why no blood was found on Martin's hands, since Zimmerman was bleeding from the nose. Di Maio said maybe Zimmerman didn't bleed right away, but he also pointed out that the rain could have washed way and affected evidence collected from Martin's hands. Morgan says this is why the hands should be secured at the scene.
Investigators also took Martin's fingerprints at the scene, which Morgan says should never be done. The decedent's hands should be handled with gloves, bagged at the scene and then fingerprinted later, in order to preserve as much evidence possible. According to standard practice, fingerprints are taken for identification after the body is transported from the scene.
Handling of the body
After Martin was confirmed dead at the scene, his body was covered with a non-specific yellow blanket and was left lying in the wet grass for hours. One of the first officers to respond to the scene reported he got the blanket out of his car and covered Martin's body after Martin was pronounced dead. It isn't clear whether that blanket had been used before and what happened to it after it was used to cover Martin's body. According to Morgan, not only could the blanket have contaminated evidence on the body, but it also could have collected trace evidence from the body that could have assisted in the investigation.
Another problem experts have pointed out about that night is that Martin's body was left in the wet grass, while it was raining, for hours. Morgan says any evidence at the scene and on Martin's body, including blood, could have been washed away, as Di Maio testified.
No 'tape lifts'
Department of Justice standards for preserving evidence on a body state that investigators on the scene should "Identify and collect trace evidence before transporting the body (e.g., blood, hair, fibers, etc.)." Morgan says this procedure is known as "tape lifts," and it's a critical part of the process that collects fiber evidence such has hairs, threads and cloth. Morgan says the reports of that night don't indicate this procedure was carried out at the scene of the shooting.
No officials present
According to the official autopsy report, no law enforcement officials were present when the medical examiner, Dr. Shiping Bao, conducted the autopsy on Martin's body. Morgan says it is essentially unheard of for no officials from local agencies such as the Florida Department of Law Enforcement, Sanford Police Department or other crime scene technicians, to witness the examination.