Investigators suspect arson in South Boston fire that killed two children

Investigators believe a fast-moving overnight fire that ripped through a South Boston townhouse and killed two young sisters, leaving another child and their mother hospitalized, was intentionally set, according to sources.

A source said police have someone in custody.

"Homicide investigators are conducting interviews, and preliminary information indicates that the fire was intentionally set," said another source.

That source said police believe the mother may have been involved in an altercation sometime before the fire. This morning, police went door to door interviewing residents. They did not name any suspects in the suspected arson.

The two sources, who work for different city departments, spoke on condition of anonymity, citing the ongoing investigation.

Firefighters were not identifying any of the victims, but neighbors said two young teenagers and a toddler lived with their mother in the single-family townhouse on 6th Street. "They're like family to me. I watched those kids grow up," said Marie Cardinale.

The survivors were hospitalized with non-life-threatening injuries including smoke inhalation, according to Steve MacDonald, Boston Fire Department spokesman.

The cause of the fire, which burnt the yellow two-story townhouse nearly to the ground, was still under investigation, MacDonald said.

"The police always are called in when we have a fatal fire," he said. "The cause is undetemined and the point of origin is inde investigation."

Firefighters arrived the house at 154 W. 6th St. three minutes after an alarm went off at 3:18 a.m., he said. It took firefighters about 20 minutes to put out the blaze, but it moved so fast that the two victims could not be saved.

"How it started and why it spread so quickly, that's part of the investigation," he said.

"Twenty units with [a total of] 75 firefighters were needed to put the fire out," MacDonald said. "The whole front of the building was on fire."

A neighbor, who declined to give her name, said the burning home had looked like "an inferno," she said. "The smoke was just blazing everywhere."

MacDonald said two adjacent homes were also damaged displacing six people, who were being helped by the Red Cross this morning.

Amelia Aubourg, spokeswoman for the American Red Cross of Massachusetts Bay, said the Red Cross responded to a call from the Boston Fire Department. We “sent trained Red Cross disaster volunteers this morning,” she said. “Upon arrival, we provided financial assistance for food, clothing, and temporary lodging for eight adults and two children…They were displaced by the fire when we arrived.”

“We will continue to do follow-up as needed,” she said. “It’s hard to assess how long recovery will take.”

Nancy Sandoval, a 29-year-old neighbor, said she is worried about how other children in the neighborhood would take the death of their friend. "They [will be] devastated when they wake up," Sandoval said.

Alex I. Oster, Globe correspondent, contributed to this report.