United Airlines is sending layoff warnings to 36,000 employees, nearly half its U.S. staff. It’s the clearest signal yet of how deeply the COVID-19 pandemic is hurting the airline industry.

United officials said Wednesday that they still hope to limit the number of layoffs by offering early retirement, but they have to send notices this month to comply with a law requiring that workers get 60 days’ notice ahead of mass job cuts.

The furloughs would include 11,000 flight attendants, 11,000 customer service and gate agents, 5,500 maintenance workers and 2,250 pilots. United officials said the notices cover 45% of its U.S. employees.