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 8% in morning trading on Monday.

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. It said the Dallas-based carrier has outdated software and operational processes, which were behind the airline's massive flight cancellations in December 2022.

"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.

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

Southwest said it was contacted by Elliott on Sunday and looks 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.