The Justice Department is gearing up for an exacting look at any proposed mergers among the nation's top health-insurance companies, amid questions inside and outside the department about whether industry consolidation could suppress competition.

The five biggest health insurers have been circling one another for potential deals. Anthem Inc. has made public a $47.5 billion bid for Cigna Corp., which Cigna has so far rejected. Aetna Inc., meanwhile, has made a takeover proposal for Humana Inc.

If the insurers succeed in striking such deals, it would leave the industry topped by three big companies, each with annual revenue of more than $100 billion. UnitedHealth Group Inc., currently the largest health insurer, also recently made a takeover approach to Aetna.

Many of the mergers under discussion have the potential to raise antitrust concerns, a senior Justice Department official said, adding that health insurers considering such deals should do a careful antitrust risk-assessment of the transactions.

Antitrust enforcers have had initial discussions about how they would approach any insurance tie-ups, and they are preparing for the possibility they could face multiple deals simultaneously, this official said.

If there were a wave of mergers at once, the department would look at the deals collectively, rather than each one in isolation. Enforcers would try to determine what effect the deals could have on the marketplace, and pursue questions about whether they would benefit consumers, the official said.

The big insurers declined to comment on the antitrust scrutiny that any potential deals in the industry might face.

In a recent call with industry analysts, Anthem Chief Executive Joseph Swedish said a combination of his company with Cigna would have "the scale to drive greater efficiency and affordability for our customers," and would be able to "accelerate improvements in the total cost of care."

A spokeswoman for America's Health Insurance Plans, the industry trade group, said health-plan combinations don't increase premiums, and that insurers' "focus is on making sure consumers have affordable coverage."

The prospect of consolidation poses high stakes for the Obama administration, whose signature domestic policy legacy is the 2010 health-care law. Some aspects of the health law were designed to increase insurance-industry competition, including marketplaces for health coverage and the creation of new nonprofit cooperative health plans around the country.

But the law also includes provisions that may have helped inspire consolidation, at least indirectly. For instance, it requires insurers to spend a certain percentage of premiums on health care, which adds to the pressure to trim administrative costs,--a benefit insurers are likely to seek from merging.

A Wall Street Journal analysis from earlier this month found some combinations of the top health insurers could damp competition in certain markets around the country.

The law also contains policies encouraging health-care providers to move to forms of payment that involve tracking the care of groups of patients, aiming to save money and improve care. Providers say they need size and resources to transform health care, one of the driving sentiments behind recent consolidation by hospital groups and other health-care providers.

Just as the Justice Department is eyeing the health-insurance side of the equation, the Federal Trade Commission, which also has antitrust enforcement powers, has raised concerns about consolidation on the hospital side, challenging several mergers.

In fact, insurers are bulking up partly to face off against the larger hospital systems in negotiations about rates and payment models.

"All of this consolidation is about bargaining power," said Glenn Melnick, a professor at the University of Southern California who specializes in health-care finance. He co-wrote a study published in the journal Health Affairs that suggested increased health-insurer consolidation could benefit consumers by pushing down hospital rates, "as long as health-plan markets remain competitive."

Some research has linked having fewer health insurers to higher insurance rates.

"There's no good evidence out there that scale is associated with lower premiums or improvements in plan quality," said Leemore Dafny, a former FTC official who is a professor at Northwestern University's Kellogg School of Management. Ms. Dafny, who co-wrote a paper tying greater competition in the health-law marketplaces to lower rates, said it isn't clear insurers would pass on to consumers the benefits of any hospital discounts they achieve.

The Justice Department has challenged, or threatened to challenge, health-insurance mergers previously. In past deal reviews, it has focused both on how a merger would affect local or regional markets as well as markets for specific insurance products.

In 2012, the department found antitrust problems with Humana's acquisition of Arcadian Management Services Inc. The companies resolved the government's objections by agreeing to divest Medicare Advantage plans—the private-health-insurance version of the government program—in 51 counties and parishes.

That same year, the department expressed concerns about WellPoint Inc.'s acquisition of Amerigroup Corp. The parties addressed those concerns by divesting Amerigroup's Virginia Medicaid managed-care business. WellPoint is now called Anthem.

In 2010, Blue Cross Blue Shield of Michigan abandoned its planned acquisition of an in-state rival when the Justice Department threatened to file an antitrust lawsuit to block the deal.

As in past government merger reviews, the input of third parties who would be affected by any insurance mergers, such as employers and health-care providers, will be important, the Justice Department official said.

Groups representing large national employers, which rely on the biggest insurers to administer their coverage, have expressed concerns publicly about a potential loss of options.

The official said the department will look closely at whether there are merger-specific cost savings produced by an insurance deal that would stimulate competition and benefit consumers, and whether any such benefits outweigh any potential risks posed by a megamerger.

Duke University law professor Barak Richman said it makes sense, with such massive deals possible, for the Justice Department to look at the combinations holistically.

"Does this create some kind of greater concentration of power in aggregate?" he said. "Are we thinking carefully about all the markets where they don't currently compete but could in future?"

A broad Justice Department examination would add to pressures on health insurers. The system of Blue Cross and Blue Shield plans is facing private antitrust lawsuits in federal court alleging that they function as an illegal cartel. The Blue Cross Blue Shield Association has said its actions are legal and benefit consumers.

Write to Brent Kendall at brent.kendall@wsj.com and Anna Wilde Mathews at anna.mathews@wsj.com

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