A "residual" amount of propane gas in a tank and a spark from a cigarette lighter ignited an explosion and fire that critically burned a man and his children in a northwestern Minnesota storage unit, state officials said Friday.

The blast and fire occurred about 12:30 pm. Monday north of Bagley on 370th Street, the Clearwater County Sheriff's Office said.

Emergency responders arrived to find the man and children ages 6 to 14 with burns, the Sheriff's Office said. The boys' sister said the brothers were with their father at the time of the incident.

The State Fire Marshal said in an update Friday that the six victims remain in critical but stable condition at HCMC in Minneapolis.

Kyra Frank identified the five children to the Minnesota Star Tribune by their first names as her brothers: Keegan, 6; Kaeto, 8; Braedynn, 10; Tannen, 12; and Zander, 14. She said their father, 55-year-old Randy Ritchie, was the man with them at the time. According to an online fundraiser, the boys suffered burns ranging "from 5% to 55%."

State fire investigators determined that a 100-pound propane cylinder, "believed to be empty, contained enough residual gas to fill the storage container to dangerous levels," a statement from the State Fire Marshal read.

Ritchie and his children entered the storage unit to retrieve toys, the statement continued. Unaware of the danger, the children entered first and did not recognize the distinct odor of mercaptan, the additive that gives propane its telltale gas smell. Moments later, a cigarette lighter's spark set off the explosion.

"This family faces a long road to recovery, but they are bravely sharing their story so that nobody else experiences such a devastating accident," the statement quoted State Fire Marshal Dan Krier as saying. "We hope this family's tragedy serves as a reminder to take precautions when storing and handling propane."

The State Fire Marshal provided the following steps that need to be taken to ensure safety around propane: Store propane tanks outside; never assume a tank is empty, and install alarms where tanks are kept. Those include smoke and carbon monoxide alarms, and explosive gas detectors.

The detectors are "a critical line of defense, especially for children who may not understand the significance of the smell," the statement emphasized. "If a mercaptan odor is present or a detector sounds, evacuate the area immediately and call 911. Do not turn appliances or lights on or off during the evacuation; even a tiny spark could ignite the gas."