An autopsy of the 75-year-old Louisiana woman who founded an African American history museum and was discovered dead in a car trunk revealed she died of "traumatic asphyxia," authorities said Monday.

Sadie Roberts-Joseph died of traumatic asphyxia, which includes suffocation, according to a preliminary autopsy conducted by the East Baton Rouge Parish Coroner's office. Baton Rouge police also confirmed Monday that the car Roberts-Joseph was found in on Friday belonged to her.

"It is with great sadness and respect we investigate any unexpected or traumatic death," the coroner's office said Monday. "When our investigation involves an innocent victim, such as Ms. Sadie Joseph, it is particularly tragic. Our condolences are extended to Ms. Joseph’s family and friends."

Sadie Roberts-Joseph, right, before the start of Stand Up for Children rally in Baton Rouge, Louisiana, in 2004. Arthur D. Lauck / The Advocate via AP file

Roberts-Joseph's niece, Pat McCallister-Leduff, told NBC News on Monday that words couldn't express the grief her family is experiencing and that her aunt "stood for everything opposite of what happened to her."

"It’s horrible that somebody would actually come to Aunt Sadie and kill her and put her in her own trunk," McCallister-Leduff said. "I just couldn’t imagine what they could have said to her or what could have happened to make them do that. She never would deserve anything like that."

The uncertainty of what happened in the two-hour window between when the family last saw Roberts-Joseph and when she was found dead is "crushing," McCallister-Leduff said. She asked that whoever killed her aunt to "just come forward."

"Just tell us, let us know who did this," she said. "Who did this?"

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.