Investigators believe Joshua Petersen, 21, had been planning a murder-suicide -- involving his infant son -- for a month.
Police say the suspect succeeded in killing young Ryker, but was stopped before killiing himself.
A candlelight vigil was held outside the home of the baby's mother, Sunday night.
Police say she was no longer involved with Petersen, but they were said to be on good terms.
Authorities believe the suspect suffers from chronic depression.