A 75-year-old Louisiana woman who founded an African American history museum was discovered dead in the trunk of a car Friday afternoon, Baton Rouge police said.

A cause of death for Sadie Roberts-Joseph is not yet known, Sgt. L'Jean McKneely said.

"Our detectives are working diligently to bring the person or persons responsible for this heinous act to justice," the Baton Rouge Police Department said Saturday in a Facebook statement.

Roberts-Joseph founded the Odell S. Williams Now and Then African American History Museum, now known as the Baton Rouge African American History Museum, in 2001, according to the Advocate. The museum is part of the New St. Luke Baptist Church campus on South Boulevard, where her brother serves as pastor, the outlet reported.

The website of the city’s visitor bureau says the museum features exhibits on African art, growing cotton, black inventors and a 1953 bus from the period of civil rights boycotts in Baton Rouge, among other things.

Baton Rouge police mourned the loss of Roberts-Joseph in its Facebook post, describing her as "a treasure" to the community."

"Ms. Sadie was a tireless advocate of peace in the community," the police department said, adding it had opportunities to work with Roberts-Joseph "on so many levels."

"From assisting with her bicycle give away at the African American Museum to working with the organization she started called CADAV, (Community Against Drugs and Violence), Ms. Sadie is a treasure to our community," whose "loss will be felt," the department said.

The NAACP Baton Rouge branch said Roberts-Joseph was a trendsetter and icon in Baton Rouge who revived Juneteenth, a holiday that commemorates the June 19, 1865 announcement by a Union Army general in Galveston, Texas, that “all slaves are free.”

State Rep. C. Denise Marcelle credited Roberts-Joseph with raising awareness of African American history.

"She never bothered anyone, just wanted to expand her African American Museum downtown," Marcelle said in a Facebook post. "I loved working with her and am saddened by her death."

Baton Rouge Mayor-President Sharon Weston Broome said Roberts-Joseph's development of the museum is "a testament of her visionary and pioneering leadership."

"She loved this city and its people," Broome wrote in a Facebook statement. "Her commitment to the cultural and educational fabric of our community is beyond description."

Broome said a reward of up to $5,000 is being offered for information that will lead to an arrest.