Berkshire Hathaway bought more shares of American Airlines and Southwest Airlines in the 2017 first quarter.

US airlines have pointed to Berkshire’s investments in the industry as evidence of its newfound strength, citing Berkshire CEO Warren Buffett’s past criticism of airlines as investment vehicles.

According to a US Securities and Exchange Commission (SEC) filing, during the first quarter Berkshire raised its stake in Dallas-based Southwest by 10% to 47.7 million shares and in Dallas/Fort Worth-based American by 8.2% to 49.3 million shares.

It also lowered its stake in Atlanta-based Delta Air Lines by 8.3% to 55 million shares. The company’s investment in Chicago-based United Airlines, in which it holds 29 million shares, did not change from the previous quarter.

Berkshire now holds stock in the four largest US airlines valued at a total of $9.2 billion.

Aaron Karp