Angelica Alvarez remained hospitalized two days after she was found unconscious with the children in the basement of their home, police Capt. Steven Mock said.
He did not give any details on what caused the deaths of the two girls and two boys, ages 2 to 8. Autopsies determined they died of asphyxia.
Mock said Alvarez, 27, was in custody at Elkhart General Hospital. He would not give details on her injuries, but said she was expected to survive.
"I know she's improving," he said.
Alvarez's husband found her and the four children — Jennifer Lopez, 8, Gonzalo Lopez, 6, Daniel Valdez, 4, and Jessica Valdez, 2 — when he returned home Tuesday night. The four children were pronounced dead at the scene.
Martha Williams, who lives behind the family's home, said she did not know Alvarez well, but that she often saw the kids playing outside, riding their bikes and tricycles on the sidewalk, and that they would wave and say hello.
"They were nice little kids, and well behaved, it seemed," she said. "I think most people are still in shock that the kids are gone."
Alvarez was hospitalized in critical condition Wednesday, but Mock said Thursday he did not know an updated condition. A hospital spokeswoman said she could not give out any information.
Mock would only say during a brief news conference that detectives made the decision to arrest Alvarez after an extensive investigation.
"Details, including physical evidence, leading to this probable cause arrest will not be answered," he said.
Bill Wargo, chief investigator for the Elkhart County prosecutor's office, said a magistrate was expected to review the case on Friday to determine whether investigators had probable cause to keep Alvarez in custody.
The prosecutor's office would then have until Wednesday to decide on formal charges, he said. He said he expects his office will seek to keep the probable cause affidavit police will file in support of the arrest sealed until a formal hearing is held.
After Alvarez's husband, Fernando Valdez, found the bodies, he ran out into the street and yelled for someone to call police, authorities said. Police officers at first thought Alvarez was also dead, but later discovered she had a faint pulse.