The two congressmen have been embroiled with health care providers over ways to address the controversial "surprise" bills. But even as major providers have launched a concentrated attack on their proposal (the No Surprise legislation) to rein in the bills, Walden and Pallone have opened a second front on the private equity firms. "We are particularly interested in your firm's relationship with any physician staffing companies and emergency transportation companies," Walden said in the press release.

Walden honed in on recent allegations that staffing firms and ambulance companies owned by the private equity sector have been running up excessive, and perhaps bogus, bills that have been landing in plan sponsors' laps. One major component of surprise bills comprises services provided outside of a plan's network that consumers thought were in-network services. The staffing services component represents in-network services that are billed at excessive rates compared to other in-network providers, Walden said.

Citing research from the American Enterprise Institute and the Brooking Institution, the release said these hidden higher costs were yet another example of surprise billing practices.

"Evidence indicates that these physician staffing firms charge significantly higher in-network rates than their counterparts, thereby driving reimbursement upwards as they enter into staffing arrangements with hospitals," the release said.

The letters were sent Sept. 16 to the private equity firms. The congressmen requested responses no later than Sept.30.

Read more: 

• What to do about surprise bills? It's not so clear-cut.
• Time for federal action against balance billing?
• Out-of-network providers vex large employer plans