Men who allegedly robbed 384 million yen ($3.5 million) from a Tokyo gold buyer in Fukuoka city on Thursday may have known his activities ahead of time and thoroughly planned the heist, investigative sources said Friday.

The robbers parked their van in the same parking lot where the victim had left his rental car before ambushing him there and absconding with the money he had just withdrawn from a nearby bank in the southwestern Japan city.

The victim, a 29-year-old employee of a precious metals shop in Tokyo, had arrived in Fukuoka on Tuesday to buy gold bullion, according to police. He has frequently visited Fukuoka on business trips, and his employer has regularly used Mizuho Bank, from which he withdrew the money on Thursday, the sources said.

Four South Koreans carrying a large amount of money were stopped at Fukuoka Airport following the heist and questioned by police for an alleged connection to the robbery, but investigators believe their involvement in the case is unlikely.

They were arrested Friday for allegedly trying to take about 730 million yen out of the country without permission from customs authorities.

The 42-year-old president of a Seoul firm told Kyodo News on Friday that the men held by the police are employees of the South Korean company and denied they were involved in the heist. He said they were carrying money they had received from a Japanese person to buy vehicles.

The heist, the fourth-largest in postwar Japan in terms of the amount stolen, occurred at around 12:25 p.m. Thursday in the parking lot across the street from Mizuho Bank's Fukuoka branch in the Tenjin district.

Two men sprayed the victim with a substance and snatched a carry-on bag containing the cash as he returned to his vehicle from the bank. They fled in a white van, likely driven by another person, according to the police.

Neither the van nor the bag has been found. The victim suffered minor injuries to his face and throat, they said.

The police have identified the van's license plate number through security camera footage, but the plate seems to have been a stolen one, the investigative sources said.

The number was registered by a man living in Fukuoka City, but the man's car was different from the van and did not have the license plate. The sources said the man is unlikely to be related to the case, they said.

In Fukuoka last July, gold bars worth around 600 million yen were stolen near JR Hakata Station in the city while being transported to a cash-for-gold store.

In Tokyo, meanwhile, a man called police on Friday afternoon saying he was robbed of a bag that had about 40 million yen inside in the Ginza shopping district, the police said.

The man told the police that a person suddenly dashed toward him from behind and took his bag. The person then got on the backseat of a motor scooter that was parked several dozen meters ahead and took off with the driver.

The man was in Ginza for a business deal and had a total of 72 million yen with him, part of which was robbed, according to the police.