Prince George’s County firefighter John Ulmschneider was shot and killed while responding to a medical emergency on April 15. A family friend of Ulmschneider described "Skillet" as a "man of great character who loved to help people." (WUSA9)

The case of a Maryland firefighter who died trying to enter a man’s home while answering a 911 call will likely go before a grand jury to determine whether charges should be filed against the shooter, who reportedly said he fired in self-defense.

While Prince George’s County officials have said preliminary findings indicate that the killing of John Ulmschneider was a mistake, authorities said Monday that investigators are waiting on results of forensics tests and interviews with at least two key witnesses recovering from the shooting as prosecutors weigh what, if any, charges are appropriate.

“This is an active investigation,” State’s Attorney Angela D. Alsobrooks said. “We will apply laws to the facts as we find them.”

The complex case probably will hinge on whether the shooter believed he was in danger when he fired and whether his actions were reasonable and necessary, according to Alsobrooks and legal experts in Maryland.

Technician Kenny Fusco affixes a black slash over the Prince George's County seals on vehicles at the Landover Hills station Saturday in honor of slain firefighter John Ulmschneider. (Fenit Nirappil/The Washington Post)

Authorities said Monday that the weapon the man used was a handgun that appears to be legally owned.

Ulmschneider, 37, and fellow firefighter Kevin Swain, 19, were shot Friday while trying to enter the Temple Hills-Camp Springs home of a 61-year-old man whose brother had called 911 asking for medics to check on the man’s welfare, police said. The 911 caller feared that his diabetic brother might have been suffering from a blackout or had a seizure, authorities said.

After knocking on the door several times and loudly announcing themselves as rescuers three times, the medics began breaking into the home because they feared there was a medical emergency inside, county authorities said.

That is when the man inside began shooting, striking Ulmschneider, Swain and the brother who called 911, authorities said.

The man who fired his weapon was released from police custody Saturday; no charges were filed against him. The Washington Post is not identifying the man because he has not been charged and police did not name him.

A sister of the man who shot the firefighters said in an interview Sunday that her brother shot in self-defense, thinking an intruder was entering.

Police say the man who fatally shot Prince George's County firefighter John Ulmschneider has been released from custody and is not facing charges at this time. (Reuters)

Swain, a volunteer for Morningside Volunteer Fire Department, has been hospitalized since Friday and is expected to make a full recovery but “has a long road ahead,” Prince George’s County Fire Chief Marc Bashoor said.

Two other first responders pulled the men who were shot away from the scene, a move Bashoor called “valiant.”

The department “lived our worst nightmare,” Bashoor said. “We prepare, we train for it, we never expect it.”

The firefighters who answered Friday’s call did “exactly what they were trained to do” given there was an “indication of a medical emergency,” but a full review of department policies will take place after Ulmschneider’s funeral Wednesday, Bashoor said.

David Gray, a law professor at the University of Maryland, said state law in self-defense shootings that result in deadly force rely on a “reasonable person standard” to determine whether the shooter should face manslaughter or murder charges.

In Maryland, “you have a right to self-defense,” Gray said. “The defendant has to show by a preponderance of the evidence that he reasonably believed he was under an imminent threat of death of severe bodily harm or that the responsive force that he used was necessary and reasonable.”

If the man who fired is not criminally charged, he could face a civil lawsuit from the slain firefighter’s family, said Isaac J. McAdams, a Maryland attorney.

“It’s still a potential wrongful death case with lower standards of proof,” said McAdams, who represents a gun range and teaches the legal portion of its class on Maryland firearms law.

Andrew Pantelis, president of the union representing Prince George’s County firefighters, said the department and the Ulmschneiders are in mourning.

Ulmschneider of St. Mary’s County was married and was the father of a 2-year-old daughter. The union has created a fundraising page to help his family at YouCaring.com.

The type of call Ulmschneider and Swain answered and the way they answered it occurs every day for firefighters and paramedics, Pantelis said.

“It is a common occurrence,” Pantelis said. “It has saved lives.”

(This file has been updated to correct Ulmschneider’s age which was misstated in an earlier version. )

Justin Jouvenal contributed to this report.