Kaiser Permanente workers in Colorado picketed on Wednesday, joining thousands of Kaiser employees striking across the country in what’s likely the largest health work stoppage in American history.

The strike – expected to last three days – signaled that Kaiser and the union failed to reach an agreement. Kaiser’s contract with the Service Employees International Union expired on Saturday.

Some 3,000 union members are expected to join today’s strike, representing roughly half of the 6,800 employees Kaiser has in Colorado.

Andrew Sorensen, a Kaiser spokesperson in Colorado, earlier said the health system would remain open during the strike. 

While Kaiser operates hospitals in other states, the health system serves more than 500,000 Coloradans through its 33 medical offices and “a network of affiliated hospitals and physicians,” according to the organization’s website.

This story is developing.