Since his arrest last week, Gen Sekine, 53, has confessed to killing one former customer over disputed payments for two African hunting dogs and is being questioned about the disappearances of other customers.
News reports Monday said Sekine had indicated during questioning that he killed three other customers with animal poison. Police declined to comment on the reported developments while the case was still unfolding.
If the suspicions are correct, the Sekine saga could become one of Japan's biggest serial murder cases, increasing fears that the country's reputation as a crime-free haven in the industrialized world is under threat.
The only serial murder in recent memory was last year and, ironically, also involved dog-lovers, when an Osaka dog trainer admitted murdering four dog owners.
A boom in rare pets as status symbols has sparked a number of disputes between breeders and buyers in Japan.
Recent shootings and gun seizures have also sparked fears that the post-war order is crumbling and that traditionally law-abiding Japan is becoming more dangerous.
The head of the National Police Agency said in November that the "foundation of public order" in Japan was under threat after statistics showed armored car robberies and gun seizures were at record levels.
His agency is set to receive 2.8 billion yen ($28 million) to help prevent gun-related violence in the next budget.
The dog breeder case has grabbed newspaper front pages and sparked news flashes on television.
Last week Sekine confessed to one murder after an accomplice, Nagayuki Yamazaki, went to the police, saying he could no longer live with the fear of being killed by Sekine. Yamazaki was himself arrested for helping dispose of one body.
Following this lead, police found human bones and teeth in a forest near Sekine's "Africa Kennel" shop in Gumma Prefecture, north of Tokyo, which they identified as the remains of Akio Kawasaki, 39, one of Sekine's customers.
In 1993, Kawasaki had paid 11 million yen ($110,000) for two Rhodesian Ridgebacks, southern African hunting dogs, after Sekine persuaded him that breeding them would be a profitable venture in Japan's pet boom.
Thinking he was on to a good deal, Kawasaki scraped the money together by withdrawing all his savings, cashing in a life insurance policy, selling his car and getting the rest of the cash from his mother.
However, one of the dogs, a female, disappeared, and Kawasaki demanded Sekine return the money. Sekine at first refused and then agreed to return 6.5 million yen. Five days later Kawasaki went missing.
Newspaper reports say Yamazaki claimed he saw Sekine making Kawasaki drink a sedative used to put down sick animals. Sekine then cut up the body and burned it in the forest, according to the reports.
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