American Airlines Jet Blue Bid about $3.6 billion to acquire its low-cost rival Soulin a move seeking to thwart the sale of that company to Frontier Group, agreed in February but not yet closed.

Spirit announced in a statement on Tuesday that It received an “unsolicited offer” from JetBlue to acquire all of its shares at $33 per share.

This offer would in principle override the pool of shares and cash that Spirit shareholders would have received under the agreement reached with Frontier last February that is still awaiting final approval.

Spirit pointed out in his note The company’s board of directors will work with its financial and legal advisors to evaluate the JetBlue proposal and decide on the option that is in the best interest of the company and its shareholders.

The statement from the low-cost airline confirmed earlier information from the New York Times, published shortly before the close of the trading session that caused Spirit shares to rise 22%, to $26.91.

WhileShares of JetBlue, which has not commented publicly, closed down more than 7% in response to news of its offering.

Florida-based Spirit and Frontier in February announced an agreement to create the fifth largest airline in the United States which, according to the companies, would improve competition in the sector by offering a real competitor to the four American giants (American Airlines, Delta, United and Southwest).

JetBlue, which focuses its operations in the east of the country, is the sixth largest US airline.