The deaths of three Baltimore firefighters who perished after being trapped in a burning vacant row home in January have been ruled homicides, police revealed Wednesday.

The criminal nature of the death came after the Bureau of Alcohol, Tobacco, Firearms and Explosives ruled the fire was “incendiary,” officials said in a press release.

The designation applies to fires that are “intentionally ignited or spread into an area where the fire should not be” or “are not necessarily intentional, but directly result from other criminal activity,” according to the bureau.

Authorities had earlier released footage of a “person of interest” in connection with the homicides, and were now hunting the suspect, the release said.

Lt. Paul Butrim, Lt. Kelsey Sadler and Firefighter/EMT Kenneth Lacayo were trapped in the vacant home when it partially collapsed during the early morning hours of Jan. 24. A fourth first responder was rescued.

“Thoughts of Lt. Butrim, Lt. Sadler, and Firefighter/EMT Lacayo have been present in our minds every step of this investigation,” said ATF Baltimore Special Agent Toni Crosby.

“The incendiary classification is an important step forward in this investigation. We will continue to work alongside our partners to ensure a complete and thorough investigation is completed.”