The death of Sadie Roberts-Joseph, an activist who founded an African American history museum in Louisiana, has been ruled a homicide, officials said Monday. The 75-year-old was found dead in her trunk last week.

An autopsy conducted by the East Baton Rouge Parish Coroner's Office said the preliminary cause of death was "traumatic asphyxia, including suffocation."

"It is with great sadness and respect we investigate any unexpected or traumatic death," the coroner's office said in a statement 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."

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Roberts-Joseph founded the Odell S. Williams Now & Then Museum of African-American History, which is now called the Baton Rouge African-American History Museum. Officials and activists in the Baton Rouge area honored Roberts-Joseph for her many years of activism and community service.

This 2004 image shows Sadie Roberts-Joseph. Arthur D. Lauck via AP

Police in Baton Rouge said Friday that Roberts-Joseph's body was discovered in the trunk of a vehicle located on a dead end street, several miles from her home. On Monday, police confirmed that the vehicle belonged to Roberts-Joseph.

The discovery left her brother Joseph Armstrong in shock. "We've never seen anything like this in our immediate family. It is really really hard," Armstrong told "CBS Evening News."

"They popped the trunk and she was in the trunk of the car," he added. "How did it happen... or why? Was it a robbery or what?"

No suspects have been named in connection with Roberts-Joseph's death. Mayor Sharon Weston Broome said the city is offering a cash reward of up to $5,000 for information leading to an arrest and indictment.

Mireya Villarreal contributed to this report.