Security has been stepped up at homes of Japanese officials after two people linked to the health ministry were killed and a third was knifed.
A former vice-minister of health and his wife were found stabbed to death in Saitama, near Tokyo, on Tuesday.
Hours later, the wife of another former official was stabbed in the capital by a man disguised as a delivery worker.
Both officials had resigned for their part in a scandal involving the loss of millions of pension records.
In 2007, ministers admitted roughly 50 million people would have to check whether or not their pension records were accurate after official records were mislaid.
The BBC reports the scandal provoked widespread anger in Japan and was partly responsible for the ruling party's defeat in elections last year.
Police said Takehiko Yamaguchi, 66, and his 61-year-old wife, Michiko, 61, were found dead at their home in Saitama, 40km (25 miles) north-west of the capital. Both had several stab wounds to their chests.
Later on Tuesday, the wife of Kenji Yoshihara, 72, another former vice-minister of health, was attacked at their Tokyo home.
Yasuko Yoshihara was stabbed several times in the chest when she opened the door to a man she thought was delivering a parcel,
Japanese media reported Mr Yoshihara was not at home at the time of the attack.
Kyodo news agency said police have stepped up security nationwide, amid fears of serial attacks against Japanese bureaucrats.