The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) Baltimore Field Division classified the fire as incendiary.

The three firefighters were responding to the fire on January 24 just before 6 a.m. when the building collapsed shortly after they entered it, local station WBFF reported.

The deaths of Lieutenant Paul Butrim and firefighter/paramedics Kenneth Lacayo and Kelsey Sadler, who was posthumously promoted to lieutenant, sparked shock and grief, while the fire itself was pinpointed as one of the deadliest in the city’s history, Newsweek previously reported.

It was not immediately clear what findings led to the incendiary classification, which the ATF defined in a press release as a “fire that is intentionally ignited or spread into an area where the fire should not be, and involves a violation of federal, state, local or tribal law.”

“Incendiary fires may also include fires that are not necessarily intentional, but directly result from other criminal activity,” the release added.

The ATF added that a person of interest in the case has been identified and that the investigation is ongoing, so the agency will not release additional information at this time.

“We would like to thank members of the community who contacted ATF and our partners with information, as well as for their outpouring of support for Baltimore City Fire Department and the loved ones of these fallen heroes,” ATF Baltimore Special Agent in Charge Toni M. Crosby said in the statement. “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.”

The ATF Baltimore Field Division declined to provide any additional information to Newsweek on the case or on what details led to the incendiary classification.

Autopsies determined that the firefighters died from injuries they received while working to extinguish the fire, WBFF reported.

A fourth responder, firefighter/EMT John McMasters, was also trapped in the vacant home when it collapsed, but others were able to pull him out. He was transported to Shock Trauma, treated and then released, the ATF release said.

After the three firefighters lost their lives, Maryland Governor Larry Hogan ordered flags to be lowered to half-staff in remembrance.

“I call on all Marylanders to honor the memories of these brave souls, and pray for their families and fellow firefighters,” Hogan said in a statement.

Two other recent fires that attracted national attention did not claim any victims. A Rhode Island family was able to escape their burning home late last month after their dog alerted them, while elderly residents had to evacuate a Dallas senior living facility last week in a fire that saw two firefighters hospitalized but no deaths.

Newsweek reached out to the Baltimore Police Department for additional information and comments.

Update 4/13/22, 3:55 p.m. ET: This story was updated with additional information and background.