Greek authorities were on Friday trying to determine how a near-defunct militant group was able to sneak at least two parcel bombs through airport security, one of which exploded at the IMF's Paris offices.
The investigation so far suggests that the booby-trapped mail sent to the IMF and the German finance ministry -- presumably by a far-left group called the Conspiracy of Fire Nuclei -- failed to raise an alarm because it contained only a small amount of gunpowder.
"The analysis so far... is that it was a small amount of gunpowder of the same type used in firecrackers... there was no detonator," Greek police minister Nikos Toskas told Skai TV on Friday, citing evidence from investigators in France and Germany.
"Clearly they are from the same source, the same organisation... the leftovers of the Nuclei", which was dismantled by police in 2011, he added.
A Greek police source told AFP that the hit, though largely symbolic, showed that Conspiracy "is an active group and there are residual members who want to make their presence felt."
A source close to the investigation in Paris said the device consisted of two tubes of black powder and a makeshift electric trigger.
But the small quantity of gunpowder used is "hard to trace", a spokesman for the airport's security company told the To Vima news website.
It was not immediately clear if the device sent to the IMF offices was in an envelope or a small parcel.
Toskas defended the screening procedures at Athens airport, saying the equipment was recently purchased from Germany and was "the best in Europe".
"The main screening is done before the plane is loaded, and this is where our investigation is focused," he said, adding that there was "no sign" that additional parcels were sent.
Fragments of Greek stamps were found at the IMF offices where the mail bomb exploded, injuring a secretary's face and hands.
Citing police sources, the Greek daily Avgi said the intended recipient was the IMF's Europe director, Jeffrey Franks.
In a further twist, the names of two senior officials in Greece's conservative New Democracy party were used as the alleged senders -- deputy leader Adonis Georgiadis and party spokesman Vassilis Kikilias, formerly a police minister himself.
Toskas said there would be a "re-evaluation" of procedures in both Greece and abroad.
IMF chief Christine Lagarde denounced a "cowardly act of violence" and said the fund would continue its work "in line with our mandate," a statement from her office said.
The Conspiracy of Fire Nuclei also claimed responsibility for the explosive device sent to the offices of German Finance Minister Wolfgang Schaeuble.
The statement was also co-authored by the Italy-based Informal Anarchist Federation (FAI).
The package was discovered on Wednesday, a day before Schaeuble was due to host his new US counterpart, Steven Mnuchin.
The Italian group has not commented on the IMF parcel.
- History of violence -
Many Greeks blame Germany and the for imposing years of public-sector cuts and policy overhauls in exchange for bailout packages needed to prop up the debt-ridden country.
The group, which is considered a terror organisation by Washington, sent mail bombs to foreign embassies in Greece and to European leaders in 2010.
The organisation denounces capitalism and consumerism, as well as police repression and worker exploitation.
Police say the name "Conspiracy of Fire Nuclei" had been used as a cover by urban militants carrying out minor arson attacks against car dealerships and police vehicles since the middle of the last decade.
But their activities escalated starting in early 2008.
In 2011, several of its members, many of them very young, were convicted of "participating in a criminal organisation" and given long prison sentences.
Three years later, the group announced its return and has since been committing sporadic attacks.