28-year-old Taimour Abdulwahab blew himself up in central Stockholm in December 2010. Despite being surrounded by people doing their Christmas shopping, no-one else was killed.
Säpo has revealed that after four years investigating the crime, it is confident that he acted alone and it will therefore close its core investigation into the suicide bombing.
"An overall assessment of all the material we gathered and analyzed has not been able to prove that any additional person in Sweden was involved in the act," said spokesperson Agnetha Hilding Qvarnström at a press conference on Wednesday.
As well as researching the case from Stockholm, the agency also carried out investigations in the UK, where Abdulwahab had previously lived.
It was unable to confrirm claims that the bomber had previously taken part in training at an extremist camp in either Iraq or Syria.
Säpo's findings mirror the judgement of the main prosecutor in the case, who also concluded that it was most likely that Abdulwahab acted without assistance.