Boston police are investigating the death of a woman found in a Mattapan apartment Saturday afternoon and have removed her three children from the home, a spokesman said.

“The incident is being investigated as an unattended/unexplained death, possibly medical in nature,’’ Officer Stephen McNulty said in an e-mail to the Globe.

Police originally said they had responded around 3:30 p.m. to a report of a possible assault and battery in the area of 2 Bismarck St. that left a person with life-threatening injuries.

Speaking to reporters at the scene, Police Commissioner William B. Evans said a neighbor reported that a door was forced open and suspected a home invasion.

But police later determined that the door was opened by family members trying to gain access to the woman’s unit at the Fairlawn Apartments.

Police found the woman dead, with three children under 5 years old alone, Evans said.

The mother was believed to be in her late 20s or early 30s. She was not identified.

The children were not injured, police said, but were being checked by medical professionals.

“They’re young, and they’re going to be fine,’’ Evans said.

It’s unknown how long the woman was dead before she was found.

“We’re not sure if it’s a natural death at this time or if there’s some trauma,’’ Evans said. “The detectives are here.’’

Calls to the Fairlawn property management office were not returned Saturday night.

A spokeswoman for the state Department of Children and Families said the agency is investigating and has taken custody of the children.

Angela Torres, a neighbor who lives in the same building, said she knew the woman.

“She was a normal girl,’’ ­she said in Spanish. “She had her kids; one of them was a little sickly. I never saw that she had any problems with anyone.’’

Torres said she believed the woman had four children, an 8-year-old boy, a pair of twin girls, and a 1-year-old son.

“Whenever we would bump into each other, we’d chat,’’ Torres said. “But outside of that, I don’t know anything about her personal life.’’

Whenever the woman or her oldest son were locked out of the building, they would knock on her door to be let in, Torres said.

But Saturday it was the victim’s aunt that knocked, something Torres found peculiar.

“They always call each other, I thought it was strange that she was knocking on my door,’’ she said.

The aunt quickly went upstairs before Torres could ask her any questions, the neighbor said.

Torres said the victim’s oldest son knocked on her door around 9 p.m. Friday to ask if she knew where his mother was.

“I thought it was strange, because he has a cellphone. . . . They’re always in communication with each other,’’ Torres said.

“I never imagined that something like this happened,’’ she said.

Adam Sennott can be reached at adam.sennott@globe.com. Aimee Ortiz can be reached at aimee.ortiz@globe.com.