A major garda probe has been launched after up to seven senior government figures were the target of an anthrax hoax last month.

The ministers and former taoiseach Bertie Ahern were posted envelopes containing a white flake-like substance and an accompanying hand-written note stating "Have a Happy Christmas". In some cases it warned them to "beware".

Among those who received the letters were transport minister Noel Dempsey, health minister Mary Harney and tourism minister Mary Hanafin.

Gardaí were also called to investigate separate death threats made against Bertie Ahern at his St Luke's constituency office in Drumcondra. It is understood a Dublin firefighter was arrested after gardaí traced three voicemails left at St Luke's in which threats were made on Ahern's life.

The man was under the influence of alcohol when he made the threatening calls and made admissions to gardaí in custody. However, it is unlikely he will be charged as the former taoiseach opted not to make a statement.

The anthrax hoax letters were sent to the ministers' Dáil offices in the run-up to Christmas. One of the ministers personally opened the envelope in his state car.

On inspecting the contents, his garda driver became concerned and called in garda experts. They were able to reassure the minister and he did not seek medical attention.

In most cases, however, the envelopes were opened by ministerial staff. Gardaí at Pearse Street – the closest garda station to Dáil Eireann – were notified in each instance.

Following analysis, the substance in the envelopes turned out to be flour-based and totally harmless.

However, sources said it was a very unnerving experience for those involved and was clearly an attempt to frighten or intimidate.

Gardaí at Pearse Street are taking the matter seriously and are trying to establish where the letters were posted.

They have analysed the envelopes and notes for fingerprints, although any letter is certain to be handled by a large number of people prior to arriving at its destination.

No suspects have yet emerged, but gardaí believe it was most likely the work of an individual rather than an organisation.

The investigation is at an early stage. The hoax follows a series of incidents involving government ministers and TDs in recent months. Windows were broken and graffiti was sprayed at Noel Dempsey's constituency office in Trim, Co Meath.

The constituency office of former junior minister Noel Treacy in Athenry, Co Galway, was also targeted.

Eggs and stones were thrown at the Galway home of another former minister Frank Fahey, while paint was thrown at health minister Mary Harney last November.

The Sunday Tribune has also learned that a Green Party minister was spat at on the street around the time of the announcement of the EU/IMF bailout.

There is no suggestion these separate events are in any way related, but they raise concerns about the safety of senior politicians. It is understood gardaí are taking a more vigilant approach to security at public appearances of ministers.