Nine more hospital trusts are to be investigated over their mortality rates, health officials said.
The NHS Commissioning Board (NHS CB) said the nine trusts have been "outliers" on the Hospital Standardised Mortality Ratio (HSMR) for two years running.
Medical director Sir Bruce Keogh is to investigate North Cumbria University Hospitals NHS Trust, United Lincolnshire Hospitals NHS Trust, George Eliot Hospital NHS Trust, Buckinghamshire Healthcare NHS Trust, Northern Lincolnshire and Goole Hospitals NHS Foundation Trust, The Dudley Group NHS Foundation Trust, Sherwood Forest Hospitals NHS Foundation Trust, Medway NHS Foundation Trust and Burton Hospitals NHS Foundation Trust.
Last week Sir Bruce announced he would be investigating five trusts which were identified by the Summary Hospital-level Mortality Indicator (SHMI) as having higher than expected death rates.
Figures suggest there were 3,000 more deaths than expected at Colchester Hospital University NHS Foundation Trust, Tameside Hospital NHS Foundation Trust, Blackpool Teaching Hospitals NHS Foundation Trust, Basildon and Thurrock University Hospitals NHS Foundation Trust and East Lancashire Hospitals NHS Trust.
The mortality ratios at the trusts were "persistently high" between July 2010 and June 2012, according to the Health and Social Care Information Centre.
The SHMI, which compares the number of patients who die following admission to hospital with the number who would be expected to die, suggests that 3,063 more people died than expected at the trusts over the two-year period.
Sir Bruce promised a "thorough and rigorous" investigation.
He said: "These hospitals are already working closely with a range of regulators. If there were concerns that services were unsafe the regulators should have intervened.
"The purpose of my investigation is to assure patients, public and Parliament that these hospitals understand why they have a high mortality and have all the support they need to improve."