South Korea has rejected North Korea's demand to conduct a joint investigation into the drones that crashed on South Korean soil late last month.An official at South Korea's presidential office said Tuesday that there could never be a case where the prime suspect was allowed to investigate evidence into his own crime.He added the South is conducting a thorough and scientific investigation to collect more evidence to prove North Korea sent the drones.South Korea's defense ministry called North Korea's offer a low-grade tactic of psychological warfare.Seoul says it was spied on by the three drones -- one of which flew over the presidential office.North Korea denied responsibility Monday, accusing the South of fabricating the latest case.The North's powerful National Defense Commission then proposed to jointly investigate all cases that Seoul blames Pyongyang for, including the Cheonan incident, the 2010 torpedo attack on a South Korean warship that killed 46 South Korean sailors.South Korea has held the North responsible following an international probe, but the North denies its involvement.The Commission asked South Korea's top presidential security adviser Kim Jang-soo to lead the South Korean team in the joint investigation.South Korea's defense ministry released an interim report on its investigation last Friday, presenting additional evidence like engine capacities, fuel-tank sizes, flight hours and designs that further point to Pyongyang as the perpetrator.Hwang Sung-hee, Arirang News.