The investigation involves the health system's acquisitions of medical practices that have made it one of Idaho's largest employers, possibly running afoul of federal and state laws intended to prevent monopolies that reduce competition.
Idaho Attorney General Lawrence Wasden in a February letter notes the health system's possible purchase of the Saltzer Medical Group as a complicating factor, and asks the health group to hold off on the purchase.
"This acquisition, if consummated, directly affects our current antitrust review," Wasden wrote.
The Nampa-based Saltzer Medical Group also has sites in Boise, Meridian and Caldwell.
St. Luke's, according to its website, has six hospitals in Boise, Meridian, Twin Falls, Ketchum, McCall and Jerome, along with more than 100 outpatient centers and clinics.
"We have grown over the last several years and it is not uncommon for the FTC to take a look at that to make sure everything is being done properly," said health system spokesman Ken Dey. "We have no doubt we've done everything right, so we're going to cooperate fully with everything they need. We haven't done (acquisitions) for market share. This has nothing to do with becoming a monopoly."
Besides St. Luke's Health System, Saint Alphonsus Health System has also been adding physician practices. But Saint Alphonsus spokeswoman Elizabeth Duncan said it is not being investigated.
Dey said that authorities haven't asked St. Luke's to turn over documents, and it's unclear how much ground the investigation will cover.
Dey noted that the health care industry has become increasingly consolidated, and that reviews of mergers and acquisitions have also become more common as a result.
The Idaho attorney general's office could opt for legal action, but Wasden in his letter said he hoped any problems could be resolved "amicably and informally without the need for litigation and court participation."