The Opening Day matchup between the Washington Nationals and New York Mets was postponed Thursday because of coronavirus concerns, throwing an expected celebration into familiar uncertainty. Hours after the decision was made, General Manager Mike Rizzo announced that three Nationals players have now tested positive this week, and a fourth player likely has, too.

The Nationals are retesting that sample, and, as Rizzo put it Thursday evening, “the different elements of the first test lead us to believe it’s a likely positive.” The Nationals announced the first case Wednesday afternoon. The next two were announced by Rizzo while the team conducted additional contact tracing for the new positive tests.

Rizzo said the two players who tested positive Thursday were already in quarantine — along with the player who probably tested positive and a staff member — after contact tracing for the first case. The initial positive result was from a test conducted Monday at the Nationals’ spring training facility in West Palm Beach, Fla. The second, third and likely fourth positive result were from tests conducted in Washington on Wednesday. Late Thursday morning, the Nationals announced they would not play Thursday or Friday “out of an abundance of caution.”

Beyond that, though, Rizzo could only convey that the Nationals’ start date remains unclear. While he spoke to reporters Thursday, players were isolating regardless of whether they had been deemed close contacts to the infected individuals. Everyone was in their houses, apartments or hotel rooms by the ballpark, waiting for some good news.

“It’s not going to happen today, and that’s the sad part,” Manager Dave Martinez said of having Opening Day postponed. “But it’s going to happen. We’re going to play in front of our fans. Right now, my concern is the safety and health of our players and staff and their family members.”

Since the test that revealed the first positive result, that infected player could have been around teammates at the facility in West Palm Beach; on a bus to the team’s charter flight to Dulles International Airport on Monday evening; on the charter flight taken by most players, coaches, front-office members and staff who were at spring training; or during a bus ride from Dulles to Washington at the end of the trip. The Nationals did not name the players who tested positive. The joint protocol between MLB and the players’ union requires consent before a person is identified for coronavirus-related absences.

MLB’s operation manual for 2021 uses the Centers for Disease Control and Prevention definition of close contact: “Being within six feet of an infected person for a cumulative total of 15 minutes or more over a 24-hour period starting from two days before illness onset until the time the individual is isolated.” Anyone who was in close contact with an infected individual has to quarantine for at least seven days. Anyone who tests positive has to isolate for at least 10 days (starting at the first sign of symptoms for a symptomatic case or the initial test for an asymptomatic individual). D.C. regulations could require close contacts to quarantine longer than MLB’s mandatory week.

One player who tested positive had a fever as his only symptom, Rizzo said Thursday. The other two were asymptomatic. Players, coaches and staff were tested again Thursday afternoon. Rizzo said he has the “utmost” confidence that the first player who tested positive did not violate MLB’s coronavirus protocols. When asked whether contact tracing had determined when and where the virus spread this week, Rizzo deflected and said: “We’re not going to go in that direction. We’re where we’re at right now, and we’re trying to get through this thing.”

After learning of the positive test late Tuesday night, players, coaches and staff took PCR and rapid tests. Of the rapid tests, one came back inconclusive, according to a person with knowledge of the results. Then two more positive results led to even more scrambling.

This is the second straight Opening Day marred by the coronavirus pandemic. Both instances have involved the Nationals. In July, hours before their season opener with the New York Yankees, Rizzo announced that outfielder Juan Soto tested positive. And on Wednesday afternoon, about 27 hours before the scheduled first pitch against the Mets, Rizzo announced this latest positive test.

At that point, Rizzo was confident the Nationals and Mets would play Thursday night. But the risk of a spread led to an MLB decision for the Nationals to pause operations. The team went the full six weeks of spring training without a positive test result. As a whole, MLB announced last Friday that it had 17 positive tests out of 72,751 conducted since mid-February.

During the 2020 season, the Miami Marlins and St. Louis Cardinals dealt with coronavirus outbreaks that led to lengthy pauses. The Marlins had 20 positive tests, including players and staff, which led to a handful of postponements. The Cardinals were derailed by more than a dozen positive tests that kept them out of action for over two weeks. The Nationals had planned to re-raise their 2019 World Series banner in front of 5,000 fans Thursday night. Now they don’t know when their season will begin.

“We’ve been through it unfortunately,” Cardinals Manager Mike Shildt said Thursday. “Hearts go out. Hope everybody’s, first of all, healthy and it doesn’t spread. We know it clearly can, quickly. Firsthand experience with that. It is unfortunate, but you are hopeful.”

“We don’t want this thing to spread any further,” Martinez explained Thursday, stressing that the Nationals will be very careful from here. “We want to make sure that everybody’s healthy.”

Should Washington play in the immediate future, the team would be without multiple players who were expected to make the Opening Day roster. On Wednesday night, when the Nationals still believed they would host the Mets on Thursday, they called up infielder Luis García, outfielder Yadiel Hernandez, catcher Tres Barrera and left-handed reliever Sam Clay from their alternate site in Fredericksburg, Va., according to people with knowledge of those moves. Rizzo said Thursday that everyone at the alternate site tested negative.

But the schedule will be shaped by future test results and contact tracing. The virus has not gone away.

“We’re still in the midst of a pandemic, and people need to still take this seriously,” Martinez said. “They really do. Unfortunately, it hit us, and we got to take care of our own now. All I can say is: Be safe, continue to wear your mask like they ask and know that this still can happen to anyone.”

Adam Kilgore contributed to this report, which has been updated.