The commissioner general of the National Police Agency has said the North Korean government was involved in smuggling several hundred kilograms of amphetamines into Japan in 2002.
"It is a case where North Korea was involved as a state," Iwao Uruma said.
This is the first time the nation's top police chief has alleged the North Korean government has been involved in drug smuggling.
In May, police arrested several people, including Woo Si Yun, 59, an alleged South Korean drug dealer, on suspicion of smuggling the amphetamines into Japan using a North Korean freighter in 2002.
The crew of the North Korean ship is believed to have dropped the drugs into the Sea of Japan off Shimane Prefecture on four occasions between June and November 2002.
Smugglers successfully recovered them using a fishing boat in three of the four cases and brought the illicit cargo to a port in Shimane.
In the fourth case, they failed to pick up the amphetamines, which drifted ashore in adjacent Tottori Prefecture, the police said.
"Relevant evidence so far prompts me to believe all the amphetamines confiscated were manufactured in North Korea," Uruma said.
"Amphetamines made in several factories in North Korea made it to Japan through several routes."
Police authorities also said Friday they have determined that the chemical composition of the amphetamines found in Tottori was all but identical to that of amphetamines they seized in the Pacific off Kochi Prefecture in 1998.
The Japan Coast Guard has concluded the North Korean boat apparently involved in the failed 1998 smuggling attempt was identical to a refitted North Korean boat that sank in 2001 after a shootout with a Japan Coast Guard vessel.