It can be grisly work - especially compared to his day job, where he runs a rehabilitation and pain management practice on Virginia Avenue in Hagerstown. But he said he finds the job fascinating and has learned to deal with the shock and distress of working with the prematurely dead.
"I almost enjoy it," said Horn, a Brooklyn, N.Y., native who came to Washington County Hospital in 1989.
Horn said his first job is to make sure the person is actually dead. Although he has never encountered it, he said live people have been mistakenly ruled dead elsewhere.
Then Horn dives into his detective work. He examines the body for a possible cause of death and also tries to determine how long the person has been dead. He said he searches for any other clues that may be helpful to criminal investigators on the scene.
Although bodies most likely will be sent to Baltimore for a full autopsy, information discovered in the first few hours can be invaluable, he said.
It's not always cut and dried, Horn said. For instance, a person burned in a fire may also have bullet wounds upon closer examination.
Even in cases where the cause of death is obvious, Horn said he still spends a great deal of time with the body, meticulously documenting his discoveries.
"It's not an issue of just locating the bullet in the head," he said.
Such documentation and attention to detail is important, Horn said, because sloppy work can ruin a conviction. He said lawyers will probe medical examiners - sometimes years after the investigation - for inconsistencies on tiny details.
"Sometimes, we don't know what's important," he said. "I was (once) asked some ridiculous question. Did I notice dirt or debris underneath the fingernails?"
Despite obvious similarities, Horn said each investigation is different. As a result, the length and methods of each can vary widely.
"Unfortunately, it's not a checklist," he said.
Horn's determinations can influence whether police continue a homicide investigation.
In one case, for instance, Horn said he examined a person who had died from a gunshot wound. It was an apparent suicide, but he said the location of the wound and the length of the weapon made it impossible for him to have pulled the trigger with his finger.
"I had one where I had trouble figuring out how he did it with a shotgun," Horn said.
Horn said he eventually concluded the man had used his foot.
It's the kind of work Horn said he never would have imagined doing while he was studying medicine at St. George's University School of Medicine. But he said he decided to give it a try about five years ago when a deputy medical examiner told a group of physicians that the county needed someone to fill a vacant post.
"Nobody raised their hand. After the meeting, I came up to him and asked what was involved," he said.
In many ways, it's a thankless job, Horn said. The deputy medical examiners are paid a small sum for each case and take turns being on call throughout the year.
Still, Horn said he finds the work rewarding - and necessary.
"Somebody's got to do it. I would hope someone would investigate my wrongful death, too," he said.