Anthem Inc. said Friday it would buy Cigna Corp. for about $54.2 billion, creating the largest U.S. health insurer by membership and accelerating the industry's consolidation from five national players to three.

The proposed acquisition, the health insurance industry's largest, comes three weeks after Aetna Inc. agreed to buy Humana Inc. for $37 billion.

The fifth national player, Minnetonka-based UnitedHealth Group Inc., remains the nation's largest health insurer by revenue.

Health insurers are finding it tougher to raise prices following the rollout of President Obama's health care law, while grappling with soaring expenses of medications including cancer drugs that can cost each patient more than $100,000 a year. Anthem said buying Cigna would help it reduce costs and negotiate lower prices with doctors and hospitals.

State insurance regulators and federal antitrust authorities are expected to scrutinize how the Anthem-Cigna and Aetna-Humana deals would affect competition for Medicare and individual and commercial insurance.

Within a few hours of the announcement, several U.S. lawmakers and a leading physicians group said they feared the pending acquisitions would hurt consumers by raising prices or limiting access to health care providers.

"The lack of a competitive health insurance market allows the few remaining companies to exploit their market power, dictate premium increases and pursue corporate policies that are contrary to patient interests," the American Medical Association said.

Under the deal, which the companies expect to close in the second half of 2016, Anthem Chief Executive Joseph Swedish would serve as CEO and chairman. Cigna CEO David Cordani would be president and chief operating officer.

In a joint conference call, Swedish told analysts Anthem had no prior discussions "at all" with regulators, but was confident about approval.

Shares of Cigna on Friday fell 5.6 percent to $145.72, far below the $188 offered in the buyout, suggesting major Wall Street concern over the antitrust risk.

"Strategically and financially it's very attractive, but they will face regulatory scrutiny," said Ana Gupte, analyst with Leerink Partners. "They also both possibly face divestitures and may have to make concessions to consumers to make the merger go through."

Cigna has 15 million members, and about 80 percent of its business is with self-insured companies which pay it a management fee, according to Leerink.

About 61 percent of Anthem's 39 million members are served through self-insured companies, while 15 percent have Medicaid coverage. Large and small group policies make up about 12 percent of its business, while Medicare Advantage accounts for 1 percent.

The combined company would have about 53 million members, surpassing UnitedHealth's 45.86 million as of June 30.