Dr. Anthony Fabio of the University of Pittsburgh's Graduate School of Public Health looked at the origin of guns recovered in suspects. (Link to study)
33% were claimed by the original owners to be stolen
44% were not legally owned by the suspect, rather it was someone elses.
14% were carrying their own firearms
Among the stolen guns, when the last place was known, 55% were stolen from homes, 18% from vehicles. 9% were from locked or secured places.