This story was originally published Dec. 27. It has been updated to reflect changes in the criminal cases mentioned.

Dallas County Medical Examiner Jeffrey Barnard and his 13 staff members perform more than 3,000 autopsies each year. Most of the time, they do their work in relative obscurity.

But occasionally there is a horrendous case that draws widespread intrigue — like the death of 3-year-old Sherin Mathews, whose decomposed body was found in a Richardson culvert in October. Her cause of death is pending. In cases like Sherin's, everyone wants to know what exactly happened and why the autopsy results take so long. After several months, an autopsy showed Sherin died from "homicidal violence" and her father was charged with capital murder.

Dr. Jeffrey Barnard (Rose Baca / Staff Photographer)

Barnard, the county's chief medical examiner since 1991, estimates he's personally performed 8,000 to 10,000 autopsies. Maybe more. He's autopsied high-profile murder victims like Aubrey Hawkins, the Irving police officer slain by seven Texas prison escapees; Mark Hasse, a Kaufman County prosecutor whose killer went on to murder the Kaufman district attorney and his wife; and Chris Kyle, the deadliest marksman in U.S. history who was gunned down by a fellow former Marine he was trying to help.

Barnard is also a professor and chief of forensic pathology at UT Southwestern Medical Center.

He recently sat down with The Dallas Morning News to answer questions about autopsies, toxicology reports and what surprises people about his job.

His answers have been edited for length and clarity.

What surprises people most about your job?

For some people, on a very basic level, it’s the fact we're real doctors.

It really takes the same number of years to do what we do as it takes to be a subspecialist in a lot of fields. I mean, I did six years of training after medical school.

Dr. Jeffrey Barnard, Dallas County chief medical examiner pointed to where injuries from bullets during former Marine Cpl. Eddie Ray Routh's capital murder trial in Erath County in February 2015. Routh, 27, of Lancaster killed former Navy SEAL Chris Kyle and his friend Chad Littlefield at a shooting range near Glen Rose. (2015 File Photo / Tom Fox)

What misconceptions do they have?

There's a misconception that when we have not finished the report and ruled on a cause of death that the body is still here. We finish the autopsy in one day.

It's just that the final decision has not been found. But we don't need the body for that. There may be a time where you have a very complicated murder that you hold them overnight to go back the next day just to go back through things.

How did you decide to become a medical examiner?

I didn't start out that way. I didn't really know much about the medical examiner's office or what they did other than the lecture in medical school, which was fascinating. We were all kind of being driven towards primary care. So I started down in general surgery, which I did like. But you don't sleep very much. I'd been married all of two days before I started a surgery internship and then basically almost didn’t see my wife for the first year. And so I switched into pathology. During the time I was in training and pathology, I was also doing emergency room work. I thought about going back to general surgery because pathology — just sitting at a microscope all day long — was not going to do it for me. And so I talked to my program director and he said, “Well, I think you ought to go look at forensic pathology.” So I did.

And that put everything together. You do procedures. You study the diseases and, ultimately, you put a picture together.

So, for me, it was what it was destined to be.

What’s the difference between cause of death and manner of death?

An English class type of explanation would be that the cause is “why they died” and the manner is “how they died.”

So, from either a traumatic or a natural disease process would be the cause. And, sometimes, it's a multilayer thing. Whereas the manner is going to be one thing. It going to either be natural or unnatural and if it's unnatural then it can be suicide, accident, homicide.

And then, if you can't make a decision, then it could be undetermined.

What can make reaching a conclusion on cause of death more difficult?

Decomposition clearly can make things harder. Burns are another one. But some of those we can still ferret through pretty well and make decisions.

Prolonged hospitalization, too. The time of the injury and the time of death are not always synonymous. And frequently they're not.

Dr. Jeffrey Barnard (left) described gunshot entry wounds on the body of Kaufman County prosecutor Mark Hasse to prosecutor Toby Shook at the murder trial of Eric Williams in December 2014. Williams was convicted and is now on death row. (2014 File Photo / Staff Photo)

How do you decide when to conduct an autopsy?

We probably autopsy 80 percent of the cases — 80 to 85 percent of the cases — because I have a high expectation of that. Lots of other offices do a lot less than that, 50 percent. I don't think that someone who's 50 years of age and drops dead is an adequate age to turn around and say “Well, it must be heart disease.” There's a lot of things that can cause a person to die. And I believe with the longer life expectancies, families have a right to know what it is.

There's unnatural cases that other places don't autopsy and I do. For example, suicides. All kinds of questions come up later. Did they have cancer? You can't answer that if you don't do the autopsy. Secondly, if they want to challenge your ruling and you didn't get around to doing an autopsy, they say, “How can you make a call if you didn't bother to do an autopsy?”

But there are about 7,000 other cases a year that are called into us that we defer that back to doctors. A lot of cancer patients, people where they may have died at home or they took them to the emergency room and the doctor says, “Yeah, I was expecting this was going to happen.”

Autopsy results can be pending for weeks or months. Why does it take so long sometimes? And what's happening during that time?

Part of it is the medical examiner's attentiveness to getting the cases done. But some cases take a while because you may be awaiting investigation from the police or medical records. You may be awaiting consultations who do their own examinations and tests. Then, they send us reports. Some of those can take a while.

What exactly are toxicology tests?

A simple answer is a drug test.

The public may think that you just squirt this stuff in there and you get an answer. But the reality is that you cannot screen for every sort of drug. It's all dependent on what the library of the instrument has in it.

We are about to begin using a new instrument that allows for a broader screening. I didn't feel that our toxicology screens were necessarily keeping up. So we changed the way that we are screening. We've expanded the validation and testing for a lot of these newer opiates because there has been more than one case that I was convinced was a drug-related death and our screen was negative and we had to send it somewhere else.

Dr. Jeffrey Barnard testifies during the January 2002 murder trial of "Texas 7" escapee Donald Newbury about the gunshot wounds that killed Irving police Officer Aubrey Hawkins on Christmas Eve 2000. Newbury was sentenced to death and was executed in February 2015. (2002 File Photo / Staff)

What skills do you use that you didn’t learn in your formal training?

It's helpful if you have a mind that thinks either the way that cops do or the way criminals do. And you're really at an advantage if you can think on both ends. It's served me in good stead trying to help police solve a case.

What cases stand out in your career?

The Nancy Dillard Lyon case from the early 1990s was an arsenic poisoning and that really was interesting because poisoning homicides are pretty uncommon. And, certainly, arsenic is like something from the early '20s.

But the reality is, if I had to pick anything, it's a large number of the cases that were unsolved. This past year, two cold cases were solved. That's because I started digging back into them with the police department. I have a whole list of cases that are unsolved that we have evidence for that may be solvable. I'm convinced that, at the minimum, there is one a year from 1990 to about 2000. Because around 2000, we started doing the kind of DNA testing we do now.

I thought there was a serial killer in the 1980s that raped and killed — strangled — elderly women. I thought there can't be more than one person who would be rape and strangle elderly women. But, sadly enough, every one had a different DNA pattern.

The results were more disturbing than I expected.

I expected a serial killer that may or may not be caught. What I found was they all had different killers.