It was the image of Emmett Till’s disfigured body, lying in an open casket for the world to see, that helped galvanize a movement. He had been beaten, but bullets also played a role in his death in 1955, fired from a weapon that was widely thought to be lost.

Now, the gun that is believed to have been used in 14-year-old Emmett’s murder — an Ithaca .45 caliber pistol — is on display at the Mississippi Civil Rights Museum in Jackson, mounted in a display case.

The museum is set to announce the acquisition on Thursday, exactly seven decades after Emmett was killed in a Mississippi barn by two white men who were angered by allegations that he had whistled at a white woman in a grocery store while visiting from Chicago. The gun is going on display less than a week after the federal Civil Rights Cold Case Records Review Board released thousands of pages of documents related to the case.

“Now we have an artifact that we can clearly pinpoint to what happened in the barn,” said Daphne Chamberlain, a civil rights historian who works at the Emmett Till Interpretive Center in Sumner, Miss. She added, “If this artifact is out there, what else is out there that has not yet been disclosed to the public or to a cultural institution for historic preservation?”

Photographs of Emmett’s body, which circulated after his mother insisted on an open casket and allowed Black journalists to document his funeral, helped to set in motion the civil rights movement.