(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.

The hedge funds believe that Centene may be able to do more to establish 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 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.