BOSTON -- A South Boston fire that claimed the lives of two young sisters over the weekend is under investigation following reports that another woman argued with the victims' mother before the blaze began.

The neighborhood where the fire occurred was mourning Acia "C.C." Johnson, 14, and her sister Sophia Johnson, 2, both of whom were killed when flames tore through their townhouse at 154 W. Sixth St. about 3 a.m. Sunday.

Investigators were still collecting evidence at the scene Monday, and a police spokesman told the Boston Globe that an arson investigation was under way.

Police were questioning several people in connection with the fire after homicide detectives spent Sunday interviewing neighbors and friends of the victims.

"We did have a number of police on scene to physically canvass the area. It's just part of the investigation and those are just some of the tools we use. In addition we had an accelerant-sniffing dog from the state police on the scene. Again, that's another tool we have where we use it and the dog did come in to work the scene," Steve McDonald of the Boston Fire Department said.

Friends said the two sisters were inseparable and when the fire broke out they ran to take cover in an attic closet on the third floor because they were too afraid to jump from the burning building.

The girls' mother was identified by neighbors as Anna Reisopolous, 34, and Acia's twin brother was identified as Raymond Johnson Jr. Both were taken to Boston Medical Center, where they were treated for non-life-threatening injuries. Witnesses said the girls' mother had to be prevented from going back into the building to save her daughters.

Acia Johnson was a student at the Patrick F. Gavin Middle School, where mourners gathered over the weekend, including a woman who baby-sat for the victims.

"I just saw the girls on Friday. I used to baby-sit the little girl. Both of the kids were very good. There is no reason that had to happen to them. You don't do that to innocent kids. I mean, come on now. You have a problem with the mother, you take it up with the mother," Sarah Bailey said, sobbing.

Fire crews responded to the blaze three minutes after the call came in, but the fire spread too quickly for the girls to escape.