Researchers have no ideas about this. It is definitely known that the tombs were made in different historical periods because they look very different. The people of Petra didn’t make any inscriptions on the tombs that would’ve helped us understand who is buried there. But they believed that the dead could eat and spend time in the other world, because they left food and different objects with their bodies.

Interestingly, in the beginning, Petra was believed to be one giant necropolis, because of the huge number of tombs. Only later did scientists find out that it was a very well-developed city.