Michael Mulgrew, the president of the city’s teachers’ union, suggested that thousands of city school workers may have been exposed to the virus or spread it over a critical two-week period through the second week in March, when schools remained open despite pressure to close them, and then for another week when teachers received in-school training for distance learning.

Asked whether he blamed those decisions for the number of dead school employees, Mr. Mulgrew hesitated. “I would like to see the science behind it, but logic would say that had something to do with it,” he said.

Thousands more have been sickened. In a one-week period, from late March to early April, there were 5,804 employees on excused leave, which includes sick leave — seven times more than during the same period last year, according to city data that surveyed a little over half of the work force.

At the Police Department, 18 percent of the officers were out sick by mid-April. At the Fire Department, 23 percent of its emergency medical responders and 16 percent of its firefighters were out sick.

“Our city is going through an unprecedented crisis and we owe an extraordinary debt to the city workers helping us,” Mr. de Blasio said in a statement. “These are our co-workers, our fellow public servants, people we know personally.

“Each loss hurts and we mourn as though they were part of our family,” he continued. “We’ll never forget those who dedicated their lives to our city, and we’re doing everything in our power to protect every single one of our public servants.”