In recent years, the museum has gathered hundreds of artifacts from other sudden, pivotal events: a suit worn in Ferguson, Mo., by a pastor protesting the death of Michael Brown; a Black Panther pin worn during the Million Man March anniversary in Washington; signs from the recent days of racial strife in Charlottesville, Va., and clothing that denounced the death of Eric Garner in New York.

“Any moment when America is debating its identity, it’s crucial to collect it,” said Lonnie G. Bunch III, the museum’s director.

In Manhattan, the New-York Historical Society, sends out its “history brigades” to events like Occupy Wall Street and the Women’s March. In Orlando, Fla., the Orange County Regional History Center hurried to collect some 5,300 items to help it record the tragedy of the Pulse nightclub shooting.

“The police were out investigating and doctors were saving lives,” said Pam Schwartz, the Orlando museum’s chief curator. “And I had a job to do, too. What I do is preserve history.”

Ms. Schwartz and her staff drove a van through the streets in the weeks after the shooting, collecting drawings, cards and other objects from impromptu memorials, and putting up signs explaining that the tributes were being taken to a museum. Later, when the crime scene investigators were finished, she returned and persuaded the owner of the nightclub to let her have for the collection a bullet-riddled door from the bathroom and a cabinet where people had hidden.

“Think of it as Abraham Lincoln’s hat,” Ms. Schwartz said. “Physical proof in 200 years that this event actually happened.”