A woman is in custody after two small children died, and another child was critically injured after they were left alone in an apartment fire in St. Louis City, Missouri, authorities said.

The fire broke out on the third floor of an occupied three-story building in the Peabody Apartment Complex in the 1100 block of Kealty Lane around 1:30 p.m, Sunday, Oct. 20, the St. Louis City Fire Department said.

Although the neighbors had tried to break the locked door, the smoke was too thick for them to go inside, authorities said.

After firefighters arrived and extinguished the flames, a 6-month-old boy was found dead in the front bedroom. Two girls, ages 4 and 5, were found in the back room injured, KMOV reported.

The two girls were taken to the hospital and listed in a critical, unstable condition. The 5-year-old died Monday. Oct. 21, according to St. Louis police.

There were no adults in the apartment at the time of the fire and fire alarms never went off, officials said.

“Even if you had working smoke alarms, there were no adults to be alerted so it wouldn’t have made much of a difference obviously,” Capt. Leon Whitener with the St. Louis Fire Department said. “The thing here is children this age should not be left home alone.”

Police said a 23-year-old woman had been taken into custody. She has not yet been charged with a crime.

The cause of the fire remains under investigation by the regional bomb and arson squads and child abuse detectives, police said.