A man is in custody after he killed an off-duty firefighter inside a home in the Holmesburg section of Northeast Philadelphia on Wednesday morning, police said.

Police responded to the home along the 4700 block of Shelmire Avenue at about 3:51 a.m. for a report of a disturbance involving a person with a gun. When they arrived, they were met by a 27-year-old man behind a locked security gate enclosing the front porch, according to investigators.

After gaining entry, police found an unresponsive man inside the home, investigators said. The 27-year-old man allegedly told police he had fatally assaulted the victim, later identified as 56-year-old Antonio Rodriguez, a Philadelphia firefighter.

Rodriguez was pronounced dead at 4:13 a.m. and the 27-year-old man was taken into custody. Police have not yet revealed the man’s identity.

Police said the man and Rodriguez knew each other though the exact nature of their relationship is unclear at this point. Police also have not yet determined an exact cause of death.

“We don’t know what occurred. There’s some kind of physical confrontation that occurs inside the property. It was not a gunshot or anything like that,” Philadelphia Police Deputy Commissioner Frank Vanore said. “It was just a physical assault. It could have been a strangulation. We are not sure until the [medical examiner] tells us.”

In addition to police, fire crews also responded to the scene.

“I can say the fire department is working closely with the Philadelphia Police Department at this time,” Daniel McCarty, executive chief of the Philadelphia Fire Department, told reporters gathered outside on Wednesday.

McCarty didn’t discuss the details of what happened. When asked why firefighters were spotted embracing outside the property as police went into the building, McCarty noted that it had been “a rough day.”

“Today’s a rough day. Definitely a rough day,” he said.

The Philadelphia Fire Department later confirmed that Rodriguez was a firefighter.