A 17-month-old girl was killed and her 4-year-old sister and their mother were critically injured Tuesday afternoon in a house fire in Bucks County, authorities said.

A neighbor noticed smoke coming from the two-story home in the 1200 block of Carolyn Drive in Upper Southampton Township and called 911 at 2:13 p.m., said township Police Chief Ron MacPherson.

Police officers attempted to enter the house through the back door, but the smoke was too heavy, MacPherson said.

Firefighters fought their way into the burning house and found the mother and her two daughters unconscious in the front upstairs bedroom, authorities said.

All three were taken to St. Mary Medical Center in Middletown Township, authorities said. The mother was transferred to Temple University Hospital, and the 4-year-old girl was taken to Children's Hospital of Philadelphia.

The 17-month-old girl died at St. Mary, hospital spokeswoman Kathleen Smith said. Tuesday night, Bucks County coroner Joseph Campbell identified the girl as Katherine Nordberg. He said an autopsy would be conducted.

Neighbors identified the mother as Sue Thomas. The other daughter injured in the fire is 4-year-old Julia Nordberg, according to neighbors. The girls' father is Eric Nordberg, who lives at the house and runs a courier business in Philadelphia, according to neighbors.

A third daughter, Bethany Nordberg, 14, was on her way home on a bus from Klinger Middle School when her father called her on her cell phone to say there had been a fire at the house, according to Bethany's friend Chelsea Coyle.

Chelsea said Tuesday night that she and her mother met Bethany at the bus stop and took her to St. Mary Medical Center to meet her father. According to Chelsea, Bethany learned from her father that Katie, as she was known, had died.

"We watched the helicopters take Sue and [Julia] to the other hospitals," Chelsea said.

Chelsea lives across street diagonally from the Thomas-Nordberg home. She said that she and Bethany rode horses and that she regularly babysat for Katie and Julia.

"I just want to know what's going to happen with my friend," Chelsea said.

The fire was placed under control at 2:44 p.m., authorities said. MacPherson said the cause of the fire was under investigation.

The property was roped off with police tape Tuesday night, but investigators could be seen looking through the second floor of the split-level home. Through a window, an upstairs bedroom appeared burned out. In another, the burned-out ceiling left the roof rafters exposed.

A minivan sat in the driveway and other parts of the front of the home looked relatively undamaged. But neighbor Joanna Kampf said the rear of the home, including the kitchen, was destroyed. Kampf said that she knew the family well and that they were planning a Memorial Day weekend party.

"I know they've been doing a lot of cooking over the last few days before the party," Kampf said.

Shocked and dismayed, neighbors were already looking for some way to help, perhaps organizing a fund-raiser.

"Maybe a little good can come of this," Kampf said. "Maybe we can all do something to help these people."

Contact staff writer Robert Moran at 215-854-5983 or bmoran@phillynews.com.