Their father, Timothy Nicholls, was upgraded from critical to fair condition Saturday morning at Denver’s University Hospital. Nicholls, 32, suffered first- and second-degree burns over 10 to 15 percent of his body, a hospital spokeswoman said. Jay Nicholls, 11, and his sisters — Sierra, 3, and Sophia, 5 — died Friday. Their mother, Deborah Nicholls, 35, wasn’t home when the fire broke out. She spent Saturday visiting her husband. "She’s holding up, considering," said Amber Penney, Deborah Nicholls’ niece who flew in from Michigan. The fire was reported at 2 a.m. by a neighbor who saw flames at the front of the house in the 4100 block of Undimmed Circle off North Carefree Circle. The Colorado Springs Fire Department and the Colorado Bureau of Investigation searched for clues Saturday while family and friends stopped by to add to the makeshift memorial in the Nicholls’ front yard. In an uncommon move, the Colorado Springs Police Department is assisting with the fire investigation. "Because of the number of deaths and not knowing if it’s going to turn into an arson," Sgt. Charles Garcia of the Major Crimes Unit said Saturday. "If it turns into an arson, it will turn into murder." The last time police assisted with a fire investigation was in July 1998, Garcia said. A 5-year-old boy died after playing with matches or a lighter. A detective shadowed fire investigators Saturday, observing evidence that was collected. Fire investigators said it could take days to determine the cause of the fire, which apparently started in the den or kitchen area. "They are going to make sure no stone is unturned," said Lt. Ted Collas, a Fire Department spokesman. "They are taking special care." A fire captain Friday said the fire was thought to be accidental. Collas didn’t want to specify Saturday if that is the case, saying that’s up to investigators to determine. Most of the house sustained fire, water and smoke damage. Investigators said a smoke detector on the basement level of the home did not have a battery. Investigators haven’t determined a damage estimate. SMOKE DETECTORS: The chances of dying in a residential fire are cut in half when a working smoke detector is installed. Place a detector outside each sleeping area and on each level of the home. Test detectors monthly and replace every 10 years. Replace batteries at least once a year. ESCAPE PLAN: Families should have and practice a fire-escape plan. Have a meeting place during a fire such as the mailbox. WHAT TO TEACH KIDS: Recognize the sound of a smoke detector. Crawl low under smoke. Cover mouths and noses in the case of heavy smoke. Touch doors before opening them. Never go back into a burning structure. Stop, drop and roll if your clothing is on fire. MORE INFORMATION: Call the Colorado Springs Fire Department at 385-7367.