Not every arson fire can be solved. It’s a harsh reality fire departments face every day.

Anchorage fire investigators say arson crimes have increased nearly threefold since 2006. They say a number of cases have gone cold.

Every fire department fears losing one of their own while battling a fierce blaze, but when a comrade is lost battling the work of an arsonist it's even more painful. AFD still hopes the public may help in solving one such case.

Within the past 30 years the Muldoon building where it happened has changed from a grocery store to a furniture store. Today it's a charter school, but its walls now conceal a mystery -- who set the Feb. 6, 1976 Bilo Grocery Store blaze at 110 Muldoon Rd. that killed AFD firefighter Charles Whitethorn, better known as "Chuck."

"We were about this far away from the building, the truck," said John Peck, a former comrade of Whitethorn's who recently revisited the scene. "That’s about 20 feet or so out, and if you think about it, they pulled the hose off the back of the truck and then they went in those doors down there at that end -- that was the last time I saw him.”

A backdraft knocked Whitethorn unconscious inside the burning building, where he died. He was only 24 years old.

"You know, Chuck was a really good guy," Peck said. "He was new to the department but he was an Air Force firefighter when we hired him, but he was a hard charger. He was just a good guy; he was a good guy to have on your -- to have on your crew."

Although more than 35 years have passed, Peck burns with intensity when asked about the fire's cause.

"This is an arson fire and I don't think there's any doubt to that: everyone that looked at the fire, the investigators, everybody said this was arson," Peck said.

In 50 years of service, Whitethorn is the only AFD member to die in the line of duty. Memories, unlike fires, can take decades to extinguish.

“Even if we were to find out (who) did it, I don't think I’d ever forget it -- I don't know how I could forget it,” Peck said.

To this day the Whitethorn arson case remains unsolved, with no leads and no new developments. Police are hoping for better results in the 2008 arson case that claimed the life of 29-year-old Brandon Wilson.

"We have had a fire where we found a body in a closet bound up with wire in a room inside a building as we were doing the overhaul,” said acting AFD Fire Marshal James Gray. “Someone was certainly concealing a crime in that case.”

Wilson was found in the apartment he shared with his wife on East Tudor Road. The fire took place just days after a similar fire in the very same apartment, and AFD and APD are still actively working the case.

In a similar unsolved arson case two years later, investigators came upon a body in a badly burned apartment off Homestead Court in Midtown Anchorage.

"When you're dealing with a fire incident you really only have one shot at this scene. Sometimes a fire impacts on the body, impacts the body and degrades the body. So what, so what may not be readily apparent is there is injury to that body."

Eleven fire units responded to the scene. Although the apartment was a total loss, the victim's husband, son and granddaughter were able to escape.

Very few details from these and other open arson cases are available to the public. Fire investigators are still holding onto critical evidence they believe will someday bring closure -- both to victims' families and to firefighters themselves, who still struggle to find justice for one of their own.

AFD says it's currently working on 24 open arson cases. Thirty other cases have been suspended for lack of new evidence.

Contact Mallory Peebles