Employees at Boeing’s St. Louis defense factories rejected a contract offered by the aerospace giant Sunday afternoon and around 3,200 workers began striking starting at midnight—the first walk out the company has faced at its St. Louis defense hub since 1996.

Boeing workers who build fighter jets went on strike Monday at midnight Central Daylight Time. About 3,200 workers at Boeing facilities in St. Louis; St. Charles, Missouri; and Mascoutah, Illinois, voted to reject a modified four-year labor agreement with Boeing,

Some 3,200 Boeing workers who build fighter jets in St. Louis and another Missouri site and one in Illinois are on strike, their union says.