Leading progressive U.S. senators are demanding Aetna come clean about its "questionable" decision to withdraw from the federal Affordable Care Act (ACA) health care exchanges after the Department of Justice (DOJ) challenged the company's proposed merger with Humana.

In a letter (pdf) sent Thursday to Aetna CEO Mark Bertolini, Sens. Bernie Sanders (I-Vt.), Elizabeth Warren (D-Mass.), Edward J. Markey (D-Mass.), Sherrod Brown (D-Ohio), and Bill Nelson (D-Fla.), probe the events that led up to last month's announcement that the insurance behemoth would pull out of 11 of the 15 states where it sells Obamacare plans.

One day after that announcement, it was revealed that Aetna had in fact directly threatened the federal government by vowing to pull out of the ACA if its proposed merger to Humana was not approved.

"Aetna's decision regarding its participation in the ACA exchanges appears to be an effort to pressure the Justice Department into approving a merger that the department has alleged violates antitrust law and has the potential to significantly harm consumers all across the country," the senators write in their letter.

They denounce such a ploy as "dangerous," "irresponsible," and "inexplicable."

"You must now answer to both your shareholders and to the thousands of Americans who trusted Aetna with their health coverage," the letter reads, before concluding with a list of questions that the senators demand Bertolini answer by next Thursday:

What exact costs will Aetna incur now that the Justice Department has challenged the merger? What costs will Aetna incur if the merger is ultimately blocked? Why did Aetna agree to a deal that included a $1 billion break-up fee? When the company agreed to this condition, did Aetna conduct an internal assessment of the risk of a DOJ challenge? When Aetna agreed to pay this fee, was Aetna aware that it would endanger participation in the ACA exchanges? What steps did Aetna take, prior to July 2016, to mitigate the risk that the Justice Department would challenge or successfully block its proposed acquisition of Humana? When did Aetna first determine that its participation in the public exchanges would be contingent upon federal government approval of its proposed acquisition of Humana? When did Aetna first inform investors that its participation in the public exchanges would be contingent upon federal government approval of its proposed acquisition of Humana? What other risks did the company disclose to investors? What criteria did Aetna use in determining the states from which to withdraw in 2016? During Aetna's April 29, 2016, Q1 2016 earnings call, Aetna said that it had a "very good cost structure" in states in which it had experienced growth in its ACA exchange population, including Florida, Georgia, and North Carolina. Why is Aetna withdrawing from states in which it had a "very good cost structure" or where Aetna has performed well in the past? How many enrollees have contacted Aetna over its decision to withdraw from the ACA exchanges? What materials and resources is Aetna making available to assist consumers in selecting new health insurance coverage?

But however the insurance company decides to respond to the senators' letter, the episode remains "the best argument for a single-payer health plan," as Robert Reich put it in August.

"[T]he real choice in the future is becoming clear," he wrote at the time. "Obamacare is only smoking it out. One alternative is a public single-payer system. The other is a hugely-expensive for-profit oligopoly with the market power to charge high prices even to healthy people—and to charge sick people (or those likely to be sick) an arm and a leg."

Indeed, as Sanders declared last month: "The provision of health care cannot continue to be dependent upon the whims and market projections of large private insurance companies whose only goal is to make as much profit as possible."