After more than 100 days of searching, fire officials said Thursday they do not know what caused the blaze that killed six young siblings in their Northeast Baltimore house and have suspended their investigation.

The department has classified the cause of the fire as “undetermined,” Fire Chief Niles Ford said during a news conference.

“Our people really wanted to find a cause to this,” Ford said. “We went in there with a passion of believing that we would find the cause.

“That’s part of the reason it took so long for me to sit here before you right now. We wanted to be as thorough as we could.”

The blaze killed six of Katie and Bill Malone’s nine children. It began in the front left corner of the family’s 2.5-story, wood-framed house on Springwood Avenue on Jan. 12 and spread so rapidly that firefighters couldn’t even tell whether it started on the inside or outside of the home, Ford said.

The collapse of the house and firefighters’ efforts to locate and remove the six children’s bodies made finding the cause impossible, he added.

“Once you move parts of that building, you upset evidence, and we did that,” Ford said. “We had to. At that point, there was no other option for us. ... The fire was so extensive from the beginning, it was a tough trail.”

Erin Malone, 8, helped her mother rescue two of her siblings, Jack, 4, and Jane, 5, from the burning home, officials said. But the house collapsed before the six other siblings could be rescued.

The deceased were Bridgette A. Malone, 11; Amelia S. Malone, 10; twins Zoe J. and Amanda C. Malone, 3; William “Billy” F. Malone IV, 2; and Daniel G. Malone, 8 months.

Katie Malone, a special assistant to U.S. Rep. Elijah E. Cummings, and the other three children were injured in the fire. Their father, Bill Malone, had been at his job at Domino’s Pizza in Cockeysville when the fire broke out, he said.

With no definitive cause, the Fire Department was unable to rule out arson, but the chief said investigators “have no reason to believe” that the fire was intentionally set.

It is uncommon for the city’s fire investigators to be unable to determine a cause, he said.

The Malone family released a statement Thursday afternoon thanking the Fire Department “for their hard work and dedication to our family.”

“We are grateful for the support we have received from the entire community in the past few months. Though it has been difficult, we are slowly recovering and will always remember the love we have received. At this time, we are asking for privacy,” the statement read.

More than a dozen stuffed animals and other items were piled around a line of wooden crosses in a small makeshift memorial Thursday in front of the now-empty lot where the home once stood.

The tragedy has haunted the firefighters who responded to the home but could not save the children, said Rick Hoffman, president of the International Association of Fire Fighters Local 734, the Baltimore firefighters’ union.

“It hit them like a ton of bricks,” Hoffman said.

“We’re trained to go through hell to get people away from death. When we can’t do it, it’s a heavy burden. It’s a truly heavy burden.”

The Fire Department and union offer counseling and other services to members affected by trauma, he said.

A 20-member Critical Incident Stress Management team is trained to recognize and help address the emotional anguish felt by firefighters and medics following such an incident.

“Anytime it involves children, it hits home even more,” he said.

“Most of the members I represent either have kids themselves or they’re uncles, aunts, or, like me, even grandparents.

“When they’ve got a chance to save a child and we don’t make it — even though the efforts are nothing less than valiant — as a group of working people, we ultimately feel that we failed that day.”

Baltimore’s firefighters are “the best in the business,” and they carry a heavy responsibility, Hoffman said.

“When we are successful, we feel unbeatable,” he added.

“When we are not successful, we feel like we’ve let someone down. It’s a tough thing to go home to.”

cmcampbell@baltsun.com

Twitter/@cmcampbell6