"The DSI has no mandate for forensic checks; therefore it is obligated to send the cases back to the police for additional evidence to explain the cause of death," he said.
The 13 victims include the Japanese cameraman shot dead on April 10 at Rajdamnoen Avenue, the six found dead at Wat Pathum in the wake of the May 19 security operations to force an end to the Ratchaprasong rally and the Italian journalist killed on May 19.
The cases were initially removed from the police investigation after the DSI designated the incidents related to the last year's violence under its jurisdiction.
When the special probes into the 13 victims hit a snag due to suspicion that the killings might involve security officials, the DSI sent the cases to the Metropolitan Police, requesting additional evidence on how the deaths occurred.
Police subsequently rejected the request on grounds that they had no jurisdiction to meddle in DSI work.
Today's decision means that the DSI will request additional evidence on the grounds that police have the mandate to furnish forensic reports.