3:21 p.m.: This post has been updated.

In response to an investigation by ProPublica, a key senator is demanding answers from federal regulators about the care patients are receiving at U.S. dialysis facilities.

Sen. Charles Grassley, R-Iowa, sent a letter Tuesday to Dr. Donald Berwick, administrator of the Centers for Medicare and Medicaid Services, expressing concern about clinic conditions and oversight described in a report published by ProPublica and the Atlantic Monthly in November.

Grassley -- among the senate's most vigilant and vocal watchdogs over health care -- is the ranking member of the Senate Finance Committee, which has jurisdiction over Medicare and Medicaid. Most dialysis is paid for by Medicare under a special entitlement created in 1972.

"I have a responsibility to the more than 100 million Americans who receive health care coverage under these programs to oversee their proper administration and ensure that taxpayer dollars are appropriately spent on safe and effective medical treatments," Grassley wrote.

ProPublica's investigation found that patients often received treatment in environments that were unsafe or unsanitary. A review of inspections conducted between 2002 and 2009 at more than 1,500 clinics turned up hundreds of instances in which facilities were cited for breaches in infection control, as well as egregious cases in which lapses in care may have led to patient injuries or deaths.

Grassley has asked for CMS to provide documents to determine the extent of such problems and to explain what steps the agency is taking to improve oversight of dialysis clinics.

He also has asked agency officials to explain why critical data about clinics' performance has not been made public. ProPublica reported that CMS has long had a trove of data about individual dialysis center performance but has not released measures such as rates of mortality, hospitalization for infection and transplantation.

ProPublica requested this information from CMS in 2008 under the Freedom of Information Act. The week before our investigation was published, the agency provided reports for all clinics from 2002 to 2010. ProPublica is preparing the data for public release.

Grassley is expected to become the ranking member of the Judiciary Committee next year but has said he will keep a seat on the finance panel, where he previously has pushed for improvements in dialysis care. He held hearings in 2000 after a critical report by the U.S. Government Accountability Office and pressed Medicare officials after the GAO reported little progress in 2003.

He has asked for CMS to respond to his current inquiry no later than Jan. 4.

CMS officials said they have received Grassley's requests and will be responding to them.