DALLAS (AP) — Activist shareholder Elliott Investment Management has bought a $1.9 billion stake in Southwest Airlines and is seeking to force out the CEO of the airline, which has struggled with operational and financial problems.

Shares in the airline rose 9% in midday trading Monday, on pace for the biggest one-day increase in four years.

In a letter to Southwest's board, the investment firm complained that Southwest's stock price has dropped more than 50% in the last three years.

The firm said Southwest failed to evolve, hurting its ability to compete with other carriers. Elliot blamed the Dallas-based carrier's massive flight cancellations in December 2022 on what it described as the airline's outdated software and operational processes.

"Poor execution and leadership's stubborn unwillingness to evolve the Company's strategy have led to deeply disappointing results for shareholders, employees and customers alike," the investment firm said in the letter, dated Monday.

Southwest CEO Robert Jordan "has delivered unacceptable financial and operational performance quarter after quarter," the letter read. It said Jordan and former CEO Gary Kelly, now the airline's executive chairman, "are not up to the task of modernizing Southwest."

Elliott is calling for executives from outside the company to replace Jordan and Kelly, and for "significant" changes on the board, including new independent directors with experience at other airlines.

Southwest said it was contacted by Elliott on Sunday and looked forward "to better understanding their views on our company."

"The Southwest Board of Directors is confident in our CEO and management's ability to execute against the company's strategic plan to drive long-term value for all shareholders, safely and reliably serve our customers and deliver on our commitments to all of our stakeholders," a spokesperson said in a statement.

Savanthi Syth, an airline analyst for Raymond James Financial, said Elliott was likely attracted by Southwest's well-known brand, leading position at many airports and strong balance sheet, among other attributes. She suggested that necessary changes shouldn't be that hard to achieve.

Southwest grew rapidly coming out of the pandemic, adding service to 18 more cities. Syth said Southwest has recognized the need to scale back those growth ambitions, although six or eight months too late, resulting in higher costs.

Southwest carries the most passengers within the United States, although Delta, United and American — all of which have more extensive international routes — are much larger by revenue. Southwest earned a profit for 47 straight years — an unmatched record in the airline business —until the coronavirus pandemic hit in 2020.

Southwest reported record revenue of $26.1 billion last year, but its $465 million profit was down from the previous two years and about one-tenth of Delta's profit.

The Wall Street Journal first reported Elliott's stake in Southwest.