In 2018, Timberline Knolls, an upscale mental health center for women that once attracted pop stars like Demi Lovato, was in trouble. At least four residents said a therapist had sexually assaulted them on the facility’s wooded campus outside Chicago.

The therapist was arrested, and the center’s corporate owner, the large national chain Acadia Healthcare, said it had made changes “to assure that it won’t happen again.”

But dangerous conditions persisted for years at Timberline Knolls, an investigation by The New York Times found, in part because of pressure to enroll more patients without hiring enough employees.

Two former residents sued Timberline Knolls last year, claiming that an aide had raped them. Acadia had hired the aide despite a criminal record that included domestic violence and gun charges.

Another resident — a child who was a ward of the state — nearly died after she overdosed on medication that had been left out in a common area, according to former staff members. And two other women died by suicide after being left unsupervised, a rare occurrence at mental health facilities.

“We were extremely understaffed,” said Cecilia Del Angel, who worked as a behavioral health aide at Timberline Knolls until last July. Several other former employees echoed that sentiment. The patient deaths, Ms. Del Angel said, were “entirely preventable.”