RENTON, Wash. — Boeing workers came back to their jobs this week after a three-week furlough intended to halt the spread of the coronavirus through the workforce. The third shift resumed at 10:30 p.m. Monday, and the first shift reported back between 5 and 6 Tuesday morning.

They returned to new hand-washing stations, to managers asking them about their health, to more signs warning of the dangers of the coronavirus — as if the workers needed the reminder, after scores of their colleagues had fallen ill during the pandemic, forcing Boeing to close some of its biggest manufacturing plants across the country.

Now Boeing has started reopening the factories in what will be a closely watched experiment in whether Americans can safely return to work, in the nation with the highest number of covid-19 casualties in the world. Before the return, the factories were scrubbed clean and adjustments were made to combat the deadly virus in places where social distancing is difficult, if not impossible.

In the Puget Sound region of Washington state, 27,000 Boeing employees returned to work. A few thousand more returned to defense plants outside Philadelphia and Columbus, Ohio. A plant outside of Charleston, S.C. remains closed for the time being.

The return left many of the employees and their families uneasy.

On Tuesday morning, the employee lot at the Boeing Renton Factory was three-quarters full as machinists and shuttle buses ferrying employees from the lot to the factory took on just one or two passengers or left empty. Drivers said there were far fewer workers than normal.

One employee, a surgical mask dangling from his wrist and safety glasses hanging around his neck, said that five of his 15 team members had not shown up for work. “A lot of guys aren’t comfortable,” said the employee, who like others interviewed spoke on the condition of anonymity because Boeing prohibits workers from speaking to reporters without authorization. “I’m not even sure if the lunch room is open.”

In the Puget Sound region alone, scores of Boeing workers had tested positive for the virus and at least one worker has died. So as the factories returned to work, union officials urged members to hold their employer accountable by speaking up and, if necessary, pulling the red card.

In soccer, a red card is used by referees to eject a player after a foul. On the Boeing factory floor, it’s used to call a foul of a different sort: to alert managers to unsafe working conditions. Traditionally, the cards have been used to alert managers to the physical dangers of working around heavy machinery or the safekeeping of dangerous chemicals. But now the union leadership is urging workers to use them to protect against the virus.

On one side of the card is a symbol of a stop sign that reads: “STOP for your safety. If you believe that continuing your work can lead to loss of life or limb, you have the right to STOP.”

On the other: “Hand this card to your supervisor or contact site/safety manager to invoke this language. You should offer to do other work which they consider safe while a decision is being made.”

David Calhoun, Boeing’s new CEO, has said repeatedly that the company’s top priority is to protect the health and safety of its employees. And the closing of the plants was the ultimate expression of that, officials have said.

In a letter to employees last week, Calhoun wrote that “colleagues returning to work will see a wide range of safety measures in place,” including “practices to enable physical distancing, such as staggered shift times, spread-out work areas and visual controls.”

Members of Congress and local officials have said they have been in close contact with Boeing as employees return to perform work deemed “essential” by the federal government. The company said some customers are still taking deliveries of airplanes. And Boeing’s Puget Sound factories also manufacture the KC-46 tanker, an aerial refueling jet, as well as the P-8 maritime patrol aircraft for the Pentagon.

In an interview, Rep. Rick Larsen (D-Wash.), whose district includes several Boeing factories, said that “most people breathed a sigh of relief” when Boeing closed its factories. He said he felt confident that the “workforce will be not just policing themselves, they’ll be policing Boeing.”

Tara Lee, a spokeswoman for Washington Gov. Jay Inslee (D), said Inslee expects “that Boeing will undertake rigorous safety protocols as they move forward.”

But Jon Holden, the head of Local 751 of the International Association of Machinists, is not convinced that is the case. He spent Monday touring the facilities with Boeing management, walking the production lines, inspecting the new hand-washing stations and the new temperature-check areas. He called those features a positive development, but he remained skeptical.

“I’m not going to say I’m satisfied,” he said in an interview. “I’m going to say that I look forward to our members ensuring that their environment is safe.”

Given the glaring problems pre-shutdown, Holden was not sure anything would be better this time, despite the company’s pledges. The fact is the work requires people to be close together. Social distancing does not apply to installing a cockpit or wiring a fuselage.

“We have thousands of people in those factories that work in close proximity to each other,” Holden said. “So for our members that go back in, we’ll be diligent about enforcing the safety rules and ensuring that there’s enough masks and gloves and cleaning supplies. But it’s not going to be easy. … It remains to be seen how it will work in practice.”

Workers interviewed outside the Renton factory Tuesday agreed.

"We're supposed to work on this plane and stay six feet apart. How are we supposed to do that?” one employee said.

Another said that managers were “doing everything they possibly can to keep us six feet apart. But when we build a wing it’s going to be difficult because of the crowded situation.”

He said he used paid vacation days during the shutdown and accomplished so many house projects that he has nothing left on his summer chore list. Nevertheless, he felt the company called workers back too soon, especially given limited global demand for new airplanes at a time when air travel is virtually grounded.

"With airlines not taking orders, they should have waited until the end of the month,” he said.

The union worked hard to negotiate a provision called Article 16.1 into the contract with Boeing that allows any employee to pull the red card and stop work.

Before the shutdown, the union praised several workers for invoking Article 16.1, highlighting their efforts in the newsletter published by the union. In one case, second-shift workers on the 777X line were sent home for feeling sick, but it was unclear whether the company had cleaned their work stations. Subsequent shifts refused to work until they had evidence that the areas had been thoroughly cleaned.

Another “pulled the red card” when workers on the 737 wing production line did not have enough protective equipment and “were required to work within six feet of each other,” according to the newsletter. The result: “Work stopped while management evaluated the situation.” Ultimately, “cases of masks were released to the manager to provide to our members.”

The union has told its members that if they are fearful they can refuse to come to work and take vacation or unpaid leave instead. “Some are high-risk by either age or underlying conditions, or they care for someone at high risk,” Holden said. “Based on their situation, they’re going to have to make a decision of whether they can go back into the workplace or not.”

While many are happy to return to work, giving them a sense of purpose and a paycheck, they know their employer faces an uncertain future. Boeing was reeling from the 737 Max crisis — the aircraft has been grounded worldwide for more than a year after two fatal crashes that killed 346 people. Then came the coronavirus, which all but killed air travel globally and evaporated demand for new airplanes.

In his note to employees, Calhoun painted a grim picture of the industry’s future, writing that the impact on the airlines “has been like nothing we have ever seen.” Airline revenue is expected to drop by $314 billion by the end of the year, he wrote, adding that “some 2,500 aircraft have been idled and passenger volume is down over 95 percent compared to last year.”

He added that “we’re in uncharted waters” and that the “impact of this global virus will change our business for years to come.”

Still, he praised the government’s $25 billion support package for the airline industry and said Boeing is looking for “the best ways to keep liquidity flowing through our business and to our supply chain until our customers are buying airplanes again.”

When that day will come is anybody’s guess, said Richard Aboulafia, an aerospace analyst with the Teal Group, a consulting firm.

“This gets to the tricky issue of denial,” he said. “I think there are a lot of people holding out some faint hope this is a V-shaped downturn. And if you still hold out that hope, then, yeah, you want be positioned for the big recovery in September. And that’s why I’m starting a unicorn petting farm for my children.”

The dire prospects were not lost on workers on the troubled 737 Max program, and one employee said it was pointless to be called back to work on a stalled project during a public health emergency. Washington has a stay-at-home order that runs through May 4, though the aerospace industry is exempt.

“We have no reason to be here,” he said. “We are not essential. … The Boeing Company is jeopardizing my health for no good reason.”

The 20-year machinist said he received pay through April 8, then applied for unemployment insurance. After inspecting Tuesday’s work conditions on a project, he walked off the job and said he would put in for retirement.

Davenport reported from Washington.