Justin Slanina’s three young sons lived with him, but they happened to be at their mother’s residence early Saturday morning when a fire broke out in his St. Paul apartment, killing the 29-year-old.

“I’m very, very thankful that we’re not mourning the loss of all of them,” said Michelle Berg, who regarded Slanina as her nephew, on Monday.

Slanina’s family and his sons, who are 6, 7 and 8, are struggling to understand his loss.

Berg and her fiancé, Bob Slanina — who is Justin’s uncle — sat down to talk with the boys.

“We told them there was an accident at their house, that there was a fire and the paramedics did everything they could to try to save their daddy, and now he’s in heaven with great-grandma Ginny and grandma Wendy,” Berg said of Justin’s mother and grandmother. “They all just started to cry and cry. It was the most heartbreaking thing to watch.”

Firefighters called to the North End on Saturday at 2:30 a.m. heard from neighbors that an adult and three children were feared to be in the unit that was on fire, said Deputy Fire Chief Roy Mokosso. They found the adult in their initial search, but no one else in secondary searches on Front Avenue near Western Avenue.

“In situations with low visibility and after having already found one victim there is a frantic sense of urgency,” Mokosso said. “The report of children possibly being missing elevates that even more.”

It turned out that Slanina’s middle son’s birthday was on Friday, so he and his brothers spent the night at their mother’s.

The cause of the fire is under investigation with initial indications that it was smoking related, according to Mokosso.

LOVED BEING A DAD, ADVENTURE AND THE OUTDOORS

Justin Slanina loved having fun with his sons, who are full of energy, Berg said.

“Even with three little boys being so close in age, he had the patience of a saint,” she said. “He knew how to make them laugh and pull them out of moods nobody else could get them out of. There wasn’t anything he wouldn’t have done for them.”

He was drawn to adventure and the outdoors.

“There’s a video of him tying an inter-tube to the back of his Jeep and flying around the snowbanks up at the cabin,” Berg said. He enjoyed snowboarding, kayaking, fishing and riding dirt bikes.

On Saturday, Slanina arrived home about 2 a.m. after being at a restaurant with friends, according to Berg. She said he was going to rest before heading out to plow snow for the landscaping company he worked for.

“He must have gone to bed and I’m assuming he slept through it all, but it wasn’t like Justin to sleep that deep — being a father he was always alert,” according to Berg.

There was a smoke detector sounding in a common area of the four-unit building when firefighters arrived, and a fire investigator is working to determine if there were functioning detectors in the unit where the fire started, Mokosso said.

Britney Ostaba, the boys’ mother, said the smoke detector in the apartment used to “randomly go off a lot. … If you would turn the oven on for five minutes, it would go off.” She said she didn’t know if it currently had a battery in it.