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Neither neighbors nor firefighters could save those trapped in the house as it burned just after 12:30 a.m.

"(Firefighters) were exhausted. They were in the front yard on one knee, and they saw me and they said, 'Chief, we did all we could,' because they knew the kids were in the house," Baltimore Fire Chief Niles Ford told CNN affiliate WBAL.

The blaze was so intense it melted a nearby car.

"I know (the deceased children) couldn't come out. They couldn't. They were trapped. And we heard that kids were yelling. They were yelling," neighbor Robert Spencer told CNN affiliate WJZ.

The mother and two boys, ages 4 and 5, remained in critical condition. An 8-year-old girl also was hospitalized and could be released shortly. She was credited with helping her mother and two brothers escape the large house in northeast Baltimore.

The ages of the other six children range from 9 months to 11 years old.

The father was at work and told authorities the home had a smoke detector and he had recently changed the battery, officials said.

The third floor of the home collapsed onto the second floor, hindering attempts by firefighters to search and rescue, Clark said.

The office of US Rep. Elijah E. Cummings identified the woman as a staff member who has worked for him almost 11 years as a special assistant in his Catonsville office.

"I am asking that our entire community pray for my staff member, Katie Malone, and her young family," Cummings said in a statement, "I am grateful to the Baltimore City Fire Department and all those who responded quickly to the devastating fire. My staff is a family and this unimaginable tragedy is shocking and heartbreaking to us all. I again ask for your prayers."

The missing children are two boys, 9 months and 2 years; and four girls: 3-year-old twins, a 10-year-old and an 11-year-old.

The cause of the fire is under investigation.

Spencer said the family was always "playing and laughing" on the porch.

He said he was devastated that he could not reach any of the children trapped inside.

"I heard the kids crying ... I couldn't save them," he said. "It was just too much fire.

"You see how big the house is. That's how big the fire was. It was everywhere; it was coming from everywhere. You couldn't do nothing."