A North Texas company linked to a national surge in costly emergency room bills has caught the attention of a leading Democratic congresswoman who is seeking answers by next month.

Sen. Claire McCaskill, D-Mo., sent a letter last week to Envision Healthcare, the parent company of Dallas-based emergency room staffing firm EmCare, according to a report Monday from Axios.

McCaskill is ranking member of the Senate Committee on Homeland Security and Governmental Affairs. In a four-page letter to Christopher Holden, head of Colorado-based Envision Healthcare, she expressed concern about the impact the company’s high emergency care costs has on consumers.

Noting EmCare’s growing footprint, she demanded details about its pricing practices and the steps the company intends to take to address concerns about exorbitant costs.

EmCare and Tennessee-based TeamHealth make up a third of the national market for companies hired to staff and manage emergency rooms for hospitals. But emergency rooms managed by EmCare have an out-of-network billing rate that's nearly five times higher, a study published in July found.

That study prompted the letter from McCaskill, who wants answers by Oct. 1.

Envision is a publicly traded company that had $3.7 billion in revenue in 2016. Hospitals contract with its subsidiary, EmCare, to recruit and evaluate doctors' credentials and coordinate their contracts at local hospitals.

"Unfortunately, these types of contracts and pricing arrangements are not widely available to the public to provide access and transparency to consumers," wrote McCaskill, who also requested a briefing be scheduled no later than Oct. 11.

Specifically, she asked Envision to share the number of hospitals EmCare contracts with to manage emergency services, what percentage of those services are billed out-of-network and the number of complaints the company has received over the last five years regarding its costs.

“It is important for Congress to understand the development of EmCare pricing practices and the steps the company intends to take to address concerns about cost and care,” the letter said.