The fire broke out around 10 p.m. Saturday, hours after a birthday party for the boys, ages 1 and 3.
The children were resuscitated on the scene in Lake Station, Indiana, and then taken to Comer Children's Hospital in Hyde Park.
"I couldn't get her - I tried to get her to go through the front, but the flames were so bad, I couldn't get in there," said John Thiel, a friend of the victim.
An emotional Thiel wondered aloud Sunday if he could have saved his friend after flames engulfed the home she shared with her two young sons.
"There's no way they could get to them, because the house was just in such a big ball of fire, it was really terrible, I mean, tragic fire," said neighbor Tom Duering.
"I came out and it was just totally engulfed, my neighbor up the street brought a sledgehammer down and he tried to bust open the door," said neighbor Catherine.
Calls to 911 began just after 10 p.m. Saturday, just hours after neighbors say the 29-year-old mother had a party for her sons.
Thiel, who attended the gathering, says he became concerned and returned to the residence when he called the home and got no answer.
"I was screaming, 'Jennifer - Jennifer, where are you at?' I didn't hear anything in the house, so then I worked my way to the back of the house and I kicked the back door in, and I could see on the kitchen floor, nobody was laying there, and then all of a sudden I heard Jennifer scream my name - 'John, please help me, please help me," said Thiel.
Firefighters arrived at the ranch-style home to find flames shooting through the roof. A frozen hydrant slowed their battle against the blaze as emergency personnel plucked a one-year-old and a three-year-old from danger.
"They gave [the first child] CPR right on the grass right behind me here, and he was breathing, I could hear him breathing - and then the second baby, they pulled [him] out," said witness Rich Yarchan.
The boys were rescued and eventually airlifted to a Chicago hospital, but firefighters were unable to reach the boys' mother, whose badly-burned body was found five feet from the front door.
"When she was screaming my name, I kept saying, 'crawl,'" said Thiel.
The Lake County Indiana Coroner is waiting to notify family members before they release the victim's name.
The state fire marshal is trying to determine the fire's cause.
"It seems like the kitchen is the place he was focusing all his investigation on last night," said Lake Station Fire Department Chief Robert James.
Officials' investigation into the fire continued Sunday evening.
An autopsy was completed on the victim. Officials say DNA and dental records were used to confirm her identity. Fire officials also say there does not appear to have been any foul play involved in the fire. One Lake Station fire suffered minor injuries while fighting the blaze.