Five fatal shootings in recent months may have been the work of a serial killer, but police in central California are baffled as to why the victims were targeted.
The five men, all shot to death alone in the dark, were not robbed or beaten before their killings and did not appear to know each other, according to officers in the city of Stockton.
Police officer Joseph Silva also said the shootings did not seem to be related to gangs or drugs.
The city of Stockton, Stockton Crime Stoppers and a local construction company owner have offered a total of $US95,000 ($150,000) for information leading to an arrest in the killings, which occurred between July 8 and September 27.
Police released a grainy still image of a "person of interest," dressed all in black and wearing a black cap, who appeared in surveillance videos from several of the crime scenes.
None of the shootings were captured by video cameras and no firearms have been recovered.
"We don't have any video of anybody holding any gun or actually committing a crime," Mr Silva said in a phone interview.
The San Joaquin County's Office of the Medical Examiner identified the victims as: Paul Yaw (killed July 8); Salvador Debudey Jr (August 11); Jonathan Hernandez Rodriguez (August 30); Juan Cruz (September 21); and Lawrence Lopez Sr.
Mr Lopez Sr was shot shortly before 2am on September 27 in a residential area just north of the city centre.
He "was just a person who was out here at the wrong place, at the wrong time, at the wrong circumstance," his brother Jerry Lopez told KXTV-TV.
"It's hard to process that this has happened."
Mr Silva confirmed the five killings were linked through ballistics evidence and the available footage, stating that all five men were shot by a handgun, though it was not clear if the same gun was used each time.
"It definitely meets the definition of a serial killer," he said.
"What makes this different is the shooter is just looking for an opportunity and unfortunately our victims were alone in a dark area."
Police say there may even be multiple people involved in the violence.
"To be honest, we just don't know," Mr Silva said.
"This person or people who are out doing this, they are definitely very bold and brazen."
Police said four of the victims were walking alone and a fifth was in a parked car when they were killed in the evening or early morning in the city of 320,000 residents.
AP/ABC
