A new report by the U.S. Justice Department says the Pennsylvania prison system employs solitary confinement far too often for inmates who exhibit mental illness or have intellectual disabilities.

The federal report, in a 28-page letter sent Monday to Gov. Tom Corbett and prison staff, said the state Department of Corrections has made some improvement in recent months but insisted that much more needs to be done.

The Justice Department says that over a recent one-year period more than 1,000 prisoners deemed to have mental health problems had been in solitary confinement for at least three months, and nearly 250 for a year or more. Pennsylvania state prisons house just over 50,000 inmates.

Corrections Secretary John Wetzel says the report doesn't fully reflect changes the state prison system's already made.