The investigation came in response to a CNN story this week that showed between 2011 to 2013, the West Palm Beach hospital had a 12.5% mortality rate
for open heart surgery, three times the national average.
"We take these allegations very seriously. CMS is actively investigating these complaints," Aaron Albright, a spokesman for the Centers for Medicare and Medicaid Services, wrote in an email. The agency is investigating because most of the patients who had heart surgery at St. Mary's were Medicaid patients.
Tenet Healthcare, which owns St. Mary's, did not respond to CNN's request for comment on the federal investigation.
Florida Gov. Rick Scott, as well as the state's Agency for Healthcare Administration and the state Department of Health, did not respond to requests for interviews about the child death rate at St. Mary's.
A federal official told CNN on Friday that "this is a serious situation."
"We want to make sure people are not exposed unnecessarily to harm," the official said.
Nneka Campbell, who lost her 10-month-old daughter, Amelia, after heart surgery at St. Mary's in 2012, said the hospital and the state should see the federal investigation "as a final warning."
"They need to close this place down," she said, adding that she was encouraged that the federal government was investigating.
CNN's story detailed the deaths of nine babies after heart surgery since the program started in 2011. The last of those deaths occurred Tuesday
. A 10th baby was left paralyzed.
The head of an expert review panel suggested last year the hospital should stop performing heart surgeries on babies, but the hospital continued to do them, and babies continued to die.