(Reuters) - Hedge funds Corvex Management LP and Sachem Head Capital Management LP have built stakes in U.S. health insurer Centene Corp and are exploring challenging its planned $17.3 billion acquisition of WellCare Health Plans Inc, people familiar with the matter said on Monday.

FILE PHOTO: Humana CEO Bruce Broussard poses for a portrait in the Manhattan borough of New York City, New York, U.S. June 22, 2017. REUTERS/Carlo Allegri/File Photo

The hedge funds believe that Centene may be able to do more to find out if another company such as Humana Inc would be interested in acquiring it, the sources said.

The hedge funds are seeking to take advantage of the fact that Centene needs to secure approval from its shareholders for the WellCare deal, according to the sources, who asked not to be identified because the matter is confidential. There is no certainty that the funds will agitate against the deal, the sources added.

“We remain as committed to our combination with WellCare today as we did when we announced it on March 27,” said Marcela Hawn, Centene’s chief communications officer.

Corvex and Sachem Head declined to comment. WellCare and Humana did not immediately respond to requests for comment.

It was unclear whether Humana would take an interest in an acquisition of Centene.

When asked during a May 1 earnings conference call with analysts about his view on Medicaid growth in light of Centene’s acquisition of WellCare, Humana’s chief executive, Bruce Broussard, said the company was “confident in our organic direction,” but that its strategy involved always looking at the market for M&A possibilities.

Centene shares rose 5 percent to $55.23 on the news. WellCare shares dropped 1.4 percent to $256.07.

Centene’s acquisition of WellCare would create a major new player in government-sponsored healthcare, serving 22 million members across 50 states. It would allow it to bulk up its government-backed Medicare and Medicaid businesses, while reducing exposure to Obamacare healthcare exchanges.

Centene’s chief executive officer, Michael Neidorff, 76, recently extended his contract to run Centene for another five years, and in a conference call with analysts said he is in good health and looking forward to continuing to run the business.

An acquisition of Centene would give Humana control over the largest Medicaid managed care organization in the country and its extensive portfolio of Medicare Advantage customers.

Centene has nearly 400,000 customers in Medicare-related plans, 8.6 million customers in Medicaid-related plans and 2.5 million commercial customers including those enrolled in Obamacare health insurance plans.

Humana is the second largest Medicare Advantage provider, with 3.7 million Medicare Advantage members and another 4.5 million in Medicare drug plans. It has about 460,000 members in state-based contracts that include programs for people eligible for Medicare and Medicaid, temporary assistance and long term support services. It has about 1.5 million commercial customers and nearly 6 million in U.S. military plans, including in dental, vision and other supplementary insurance.

A Humana tie-up with Centene would face limited antitrust hurdles because there is relatively little overlap in the markets that the two companies operate in.