Liberal economist and New York Times columnist Paul Krugman said it's possible a major health insurer withdrew from the Obamacare health exchanges as retribution for federal authorities blocking the company from merging with another.

Earlier this week, Aetna, one of the country's largest health insurance providers, announced it would be withdrawing from the private insurance exchange markets that are a cornerstone of how the new healthcare law operates. The company cited decreased revenue as the main reason.

In an op-ed for the Times on Friday, Krugman suggested Aetna may have been motivated by bitterness rather than business.

"Some insurers are responding [to new financial burdens] by hiking premiums, which were initially set well below what the law's framers expected," he said. "And some insurers are simply pulling out of the system. In Aetna's case there's reason to believe that there was also another factor: vindictiveness on the part of the insurer after antitrust authorities turned down a proposed merger."

Krugman's column linked to a Huffington Post story that said Aetna's decision "was directly related to a Department of Justice decision to block the insurer's potentially lucrative merger with [insurer] Humana."

The details of the blocked merger, however, are based on a letter Aetna's CEO Mark Bertolini sent to the DOJ this summer, in which he was responding to an inquiry from the department about how the blockage would affect the company.

"It is very likely that we would need to leave the public exchange business entirely," the letter said, "and plan for additional business efficiencies should our deal ultimately be blocked."

Critics of Obamacare have long maintained that it would place a heavy financial burden on private insurers, moving them to increase premiums and eliminate some insurance plans, both of which have happened since the law took effect.

Krugman said in his column, though, that "most of the news about health reform since then has been good."