Japanese police were Wednesday investigating suspected `terror` attacks on former top pension bureaucrats after three people were stabbed on their doorsteps, two of them fatally, officials said.

Police were on high alert guarding top bureaucrats a day after a 61-year old former deputy welfare minister and his wife were found dead with stab wounds at their home in Saitama, north of Tokyo.

The wife of another 76-year-old former vice welfare minister was also stabbed and seriously wounded on her doorstep late Tuesday by a man pretending to be from a parcel delivery service.

Government officials said attacks could be politically motivated.

`They must be terror attacks,` said the secretary-general of the ruling Liberal Democratic Party, Hiroyuki Hosoda, noting that two former bureaucrats were targeted.

Both officials once served as directors at the welfare ministry`s troubled pension division, raising the possibility that the attacks are linked to the loss of millions of pension payment records.

`A connection between the two incidents has not been established but police are making investigations taking into account the possibility of serial terror attacks,` chief government spokesman Takeo Kawamura said.

`We take the cases seriously as they happened amid various problems in health and welfare administration,` he told reporters.

The Social Insurance Agency, which operates under the ministry, admitted last year to losing millions of payment records due to years of mismanagement, angering many elderly Japanese.

The ministry has submitted a list of current and retired bureaucrats to police for their protection. It also tightened security at its building in central Tokyo, requiring ordinary visitors to go through metal detectors.

Prime Minister Taro Aso cancelled his morning walk because of security concerns.

`Police need to investigate these cases taking into account the possibility that they may be linked,` National Police Agency official Tsuyoshi Yoneda said.

`If they are, the most significant thing is that we never allow the third attack to occur,` he added.

Pensions mismanagement was seen as a major factor in the ruling Liberal Democratic Party`s defeat in the July 29 upper house elections last year.

Aso`s predecessor Yasuo Fukuda abruptly quit the post in September after his popularity tumbled, partly due to the lost pension records.