Where might mitochondria have originated?

a mitochondrion (plural mitochondria) (from Greek μιτος or mitos, thread + κουδριον or khondrion, granule) is an organelle, variants of which are found in most eukaryotic cells.[1] Mitochondria are sometimes described as "cellular power plants," because they convert organic materials into energy in the form of ATP via the process of oxidative phosphorylation. Usually a cell has hundreds or thousands of mitochondria, which can occupy up to 25% of the cell's cytoplasm. Mitochondria have their own DNA and may, according to the endosymbiotic theory, be descended from free-living prokaryotes that were closely related to rickettsia bacteria