A handwritten note reading "Sorry, my first kill was clumsy" has reportedly been found at a murder scene.
Married couple Golam and Shamima Rabbi were discovered shot dead at their home in San Jose, California, on 24 April.
Concerned friends visited their home after not seeing them for a few days.
Police have not commented on the existence of the note, but local broadcaster ABC7 was told about it by one of the people who discovered the couple.
“Both victims were shot at least once," San Jose Police Department said in a statement.
"The motive has not been determined. There is no suspect information at this time."
Local residents reacted with shock to the murders and said the Rabbis, originally from Bangladesh, were “humble, polite and peaceful” people. They leave behind two sons, aged 17 and 23.
“That's why it's so difficult to reconcile to the tragic, violent end to such simple, gentle souls,” Hasan Rahim, a member of the Evergreen Islamic Centre told the broadcaster.
“We ask Allah to forgive the deceased, shower his mercy on them, and make their graves a garden from paradise. In addition, we extend our deepest condolences to the family and ask Allah to grant them strength and patience during this most difficult time.”
Another friend, Faisal Yazadi, said: "We are surprised that they are no longer with us and especially going away in such a manner is something we just can't come to grips with.”
Police have ruled out religious hatred as a motivation for the attack.
"Based on the information we have at this time, we do not believe this incident is a hate crime or that the public is in any immediate danger as a result of the suspect's actions," a police source told local newspaper Mercury News.
Police believe the murder "was not a random act of violence and the suspect was someone familiar to the family.”
