All six remain sedated in critical condition at a Twin Cities burn unit, but family members said they are responsive and doctors are optimistic.

Five boys and their father are recovering from serious burn injuries after a propane tank exploded in Bagley. According to the Minnesota State Fire Marshal’s Office, the accident happened Monday when a father and his sons opened a storage container to get some toys.

A “residual” amount of propane gas in a tank was all that was needed for a flicker from a cigarette lighter to ignite an explosion and fire that critically burned a man and his children in a northwestern Minnesota storage unit,