FERGUSON, MO. -- A fire that killed five members of a Missouri family was intentionally set by the mother, police said.

St. Louis County police announced late Wednesday that a preliminary investigation ruled that the deaths of Bernadine Pruessner, her twin 9-year-old daughters Ivy and Lillie Pruessner, and her children 5-year-old Jackson Spader and 2-year-old Millie Spader was a murder-suicide.

Police Sgt. Tracy Panus said in an email that investigators believe that Bernadine Pruessner, 39, set fire to a mattress. "A note was also left stating Bernadine's intentions to take her life and the lives of her children," Panus said.

Responding firefighters found the home in Ferguson engulfed in flames at 4:23 a.m. Monday. Neighbours had tried to save the family, but the fire was too intense.