He said the boys were knocked unconscious from the smoke and likely died before the flames hit their bodies.
"Smoke inhalation is usually the main cause of death. The smoke gets you first," Zurolo said.
The homeowner, Maxwell B. Hope Jr., 54, his son, Max Hope, 17, and a 15-year-old boy escaped the fire.
Zurolo said the five teenagers were sleeping in the same second-story room when the fire started.
He said, "Right now, we know four of the five young men were smokers. Investigation has shown at least one of the deceased was smoking prior to the fire."
Zurolo did not know if there were working smoke detectors elsewhere in the house.
"There was a smoke alarm in the stairwell, whether or not that was working, I don't know," he said.
Zurolo said fire marshals are awaiting toxicology reports - tests for blood alcohol content and substances - for the deceased from the Maryland Office of the Chief Medical Examiner.
The investigation is not complete, he said.
Zurolo said, "This is such a tragic incident that such young men have to lose their lives at such a young age. If people get anything out of it, smoke in an approved area, whether it be in your kitchen or outside. Smoking inside or in your bedroom area is certainly not advisable."
In this incident, Zurolo said, "Somebody was smoking and, for whatever reason, they may have fallen asleep and whatever was nearby, it ignited."
He said he was not sure what ignited, but said "it probably ignited the bedding or a combustible material nearby."
Zurolo said one of the surviving teens smelled the fire first.
"It's hard to say what took place in that room," he said. "They may have tried to extinguish the fire at the same time notifying Mr. Hope and at the same time trying to (rouse) these other boys ... I would assume the smoke and possibly the flames became too intense."
Zurolo said, "Make sure you have working smoke alarms. That's the most important thing out of this whole incident."