Police were alerted to the discovery by the flat’s owner. The evidence was taken to a crime laboratory for analysis, a police official said.

The anti-terrorism unit is examining if the case is related to a bomb that went off as it was being made in an apartment on Oct 31, an incident it has linked to a revival of anti-establishment guerrilla group activities.

A senior government official told Reuters the arms cache was probably an old hideout of an inactive urban guerrilla group and may not be directly linked to the recent bomb explosion.

So far, four people face charges that include setting up and participating in a terrorist group and illegal possession of explosives over the blast. All have denied wrongdoing.

On Monday, police arrested a 31-year-old man, a self-proclaimed anarchist.

A 31-year-old suspect and a 30-year-old woman, a Swiss resident, have also been detained pending trial. The 33-year-old woman who was injured in the blast is also under arrest.

The original target of the bomb remains unknown.

Police have analysed CCTV footage and believe that the group, including the 36-year-old man who was killed, had been planning an attack on the day of the blast.

Bomb, arson and gun attacks against politicians, police, judges and embassies were frequent after the police killing of a teenager in December 2008 in the run-up to Greece’s decade-long debt crisis. Attacks have eased in recent years.