South Korea rejected on Tuesday North Korea's demand that the rival Koreas conduct a joint investigation into drones that Seoul saidPyongyang sent to the South for spying purposes. South Korea said Friday that it believes North Korea was the origin of three unmanned aerial vehicles that crashed in the South, though conclusive evidence has yet to be found, adding it will continue investigations into the drones in a scientific manner to ensure that the drones belonged to North Korea. "In no case would a suspect be allowed to investigate evidence of hisown crime," a presidential official told reporters. His comment came hours after the North's powerful National Defense Commission accused South Korea of fabricating the latest case to shift the responsibility for the acute inter-Korean relations to the North. North Korea "remains unchanged in its stand to jointly investigate all cases, including the Cheonan case, as (South Korea) claims the North was involved in them," the commission said, referring to the South Korean warship that sank near their disputed western sea border, killing 46 South Korean sailors. A South Korean-led international investigation found that North Korea torpedoed the warship on March 26, 2010, though Pyongyang has denied its responsibility.