Spirit Airlines said Monday it still supports Frontier Airlines’ $2.9 billion takeover bid for the airline, saying it was more likely to win regulatory approval than JetBlue’s $3.6 billion competing bid.

Spirit said antitrust regulators are unlikely to approve JetBlue’s offer because of JetBlue’s alliance with American Airlines in the Northeast, a deal the Justice Department is suing to block.

“We have a hard time understanding how JetBlue could believe” that the Justice Department or a court would let JetBlue settle with American and then buy Spirit, thereby eliminating the nation’s largest low-cost carrier, Spirit’s board said in a statement. a letter to the directors of JetBlue.

JetBlue rejected Spirit’s view, especially after promising last week to make concessions designed to secure regulatory approval for its offer. JetBlue’s CEO appeared to raise the possibility of a hostile takeover bid.

Shares of Miramar, Florida-based Spirit sank more than 9%. Shares of New York-based JetBlue gained nearly 3%, while shares of Denver-based Frontier fell 4%.

The development was a reversal from last month, when Spirit said that after speaking with financial and legal advisers, its directors believed JetBlue’s offer could “reasonably” turn out to be the better of the two offers.

Spirit said its board continues to unanimously support Frontier’s February offer and sees it as the best way to maximize value. The airline anticipates an agreement with the closure of Frontier in the second half of the year.

Spirit and other competitors opposed the JetBlue-American cooperative venture in Boston and New York, called the Northeast Alliance, or NEA, long before Frontier’s February bid to buy Spirit.

etBlue tried to satisfy regulatory concerns by offering to sell Spirit’s airport gates and takeoff and landing slots in New York and Boston and perhaps Fort Lauderdale, Florida. However, Spirit’s board of directors said Monday that the revised offer is unlikely to placate regulators because the revised offer still “makes it clear that JetBlue is unwilling to end” the partnership with American.

A Spirit-Frontier merger would combine the nation’s two largest low-cost carriers and create America’s No. 5 carrier. While Spirit and Frontier are similar “ultra-low-cost” carriers, JetBlue operates with a business model that more closely resembles the Big Four: American, Delta, United and Southwest. JetBlue would absorb Spirit and eliminate a budget airline that regulators believe helps keep ticket prices low.

JetBlue on Monday repeated the argument that its offer is better for Spirit shareholders: It would pay them $33 per share in cash compared to Frontier’s cash-and-stock offer of $22.42 per share, and JetBlue’s offer it was sweetened to include a $200 million breakdown up to the fee if the deal fails.

“We hope that Spirit’s board of directors now recognizes that ours is clearly a superior proposition and engages with us in a more constructive manner than to date,” said JetBlue CEO Robin Hayes.

Hayes was far more direct, even threatening, in a five-page letter last week to Spirit Chairman Mac Gardner and CEO Ted Christie. Hayes wrote that his airline divestment promises should comfort Spirit leaders about JetBlue’s ability to win antitrust approval.

“While we would certainly prefer to negotiate a transaction with you, if you continue to refuse to constructively engage with us so that we can deliver this value to your shareholders, we are actively considering all other options available to us,” Hayes wrote.

Frontier CEO Barry Biffle said last week that regulatory review of a Frontier-Spirit combination is “already underway and many months ahead of any alternative.”

When pressed by an analyst about why Frontier hasn’t campaigned more aggressively and publicly for its offer, Biffle said, “We’ve been pretty clear” about how Frontier views the benefits of its offer. “I don’t think we have to keep repeating it.”

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