Massachusetts Sen. Elizabeth Warren and a handful of fellow Democratic senators are demanding Aetna explain its decision to abandon Affordable Care Act insurance exchanges next year in more than two-thirds of the counties where it now sells the coverage.

The senators are sending a letter to Aetna CEO Mark Bertolini on Thursday.

The senators question whether the Connecticut-based insurer was motivated by the Justice Department's decision to challenge Aetna's proposed $37 billion merger with Humana.

They also want to know what criteria Aetna used to determine which states to withdraw from.

Sens. Edward Markey of Massachusetts, Bernie Sanders of Vermont, Sherrod Brown of Ohio and Bill Nelson of Florida also signed the letter.

Bertolini has said the insurer could grow its exchange business "should there be meaningful exchange-related policy improvements."