BOSTON -- Two children from a South Boston family were trapped by smoke and flames and killed in a fast-moving fire early Sunday morning, as the girls' brother screamed at them to jump to the street below.

Police were investigating whether it was arson.

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“She was like, ‘Help, help,’ and he told her, ‘Throw yourself,’ and she said, ‘I can’t,’” a neighbor said.

NewsCenter 5’s Shiba Russell reported the fire broke out about 3 a.m. while the family was asleep in their home at 154 W. 6th St. in South Boston. Acia Johnson, 14, and her sister Sophia Johnson, 3, were on the top floor of their triple-decker.

"I went outside and flames were shooting everywhere," said neighbor Paul Chase.

The two girls were later found by firefighters in a closet on the top floor.

One source told NewsCenter 5 the older sister was crouched around her younger sister to protect her from the flames.

The girls' brother Raymond Johnson, 14, and mother Anna were able to escape, and were taken to Boston Medical Center. The brother was released Sunday afternoon. The mother was in critical condition.

“I held the mother, she was really distraught and hysterical,” said family friend Danny Zyskowski. “It’s so tragic that something like this happened to such a beautiful woman.”

Dozens of police officers were scouring the scene and nearby streets five hours after the fire broke out, looking for the cause of the fire.