SACRAMENTO  (AP) — A federal judge on Wednesday rejected California's call for an immediate end to federal oversight of medical care in state prisons, ruling that the state has yet to prove that it's ready to retake control.

U.S. District Judge Thelton Henderson, of San Francisco, instead proposed a gradual transition back to state control. He set no deadline in the four-page ruling, which comes six years after he appointed a receiver to run inmate health care operations.

The transition could take at least a year, said Don Specter, director of the nonprofit Prison Law Office, which filed the lawsuit.

Henderson's order came after Gov. Jerry Brown's administration and attorneys representing inmates could not agree on ending a receivership that has cost taxpayers billions of dollars and forced a shift of lower-level criminals from state prisons to county jails.

Instead, the two sides submitted conflicting recommendations to Henderson earlier this month. Their recommendations show that the receiver "has made significant progress in improving the delivery of medical care in California's prisons," the judge wrote.

But they also reveal lingering problems over stalled construction projects and over how to measure when prisons are providing adequate health care.

Henderson concluded that while Brown's administration said it is ready to retake control of inmate medical care within 30 days, "the record does not contain sufficient evidence to support that assertion."

He ordered the federal receiver, J. Clark Kelso, to continue gradually turning over his responsibilities to the state Department of Corrections and Rehabilitation. He also told Kelso to work with court experts to decide how to evaluate medical care provided by the prisons.

Kelso's spokeswoman, Nancy Kincaid, could not estimate how long the process would take. Henderson told both sides to comment on his transition plan by July 20.

"The bottom line is he seems like he wants to ensure that no matter what happens, the care doesn't get any worse and hopefully gets better," Specter said. "How this is managed is very important because we're talking about life and death issues here."

Corrections Department spokesman Jeffrey Callison said the state still believes the receiver is no longer necessary, despite the judge's ruling. But he said officials are encouraged that the judge is ordering Kelso to continue returning health care authority to the state.