According to the statement, there is no indication "at this point in the investigation" that Earnest was part of an organised group.
Police believe that the shooting suspect was acting alone "without outside support in carrying out the attack".
Earlier in the day, it was reported that the three people wounded in the shooting have been released from hospitals. Rabbi Yisroel Goldstein was shot in the hand. Two worshippers — 34-year-old Almog Peretz and 8-year-old Noya Dahan — suffered shrapnel wounds, an AP report says.
The author of the manifesto, who reportedly identified himself as a 19-year-old nursing student with the same name as the detained suspect, claims to have been motivated by the Christchurch mosque carnage in March and the shooting at the Pittsburgh Tree of Life synagogue last year.
San Diego Sherriff William Gore has revealed that Earnest was also under investigation in connection with a case of arson at an Escondido mosque last month.
The shooting incident, resulting in multiple injuries, took place on 27 April at The Chabad Synagogue in Poway, California.
READ MORE: 'Jews Deserve Hell': What is Known So Far About California Synagogue Shooter