Argentine police are investigating a spectacular assault on a Buenos Aires bank amid the trial of those accused of pulling off the 2006 heist dubbed the 'robbery of the century'.

The robbery was staged last weekend, but the police could not get into the bank until Monday and authorities still have not collected all the evidence at the crime scene, a branch of Banco Macro barely 200 meters from the Argentine Congress.

Sources confirmed Thursday that dozens of people were under suspicion, from former and current bank employees to security personnel and outside contractors who recently performed work of one sort or another at the bank.

The caper began Saturday night, when a group of masked men entered an industrial-safety school operating one floor above the bank and overpowered the guard and his son, locking them up in a neighbouring room.

The thieves then went into the school's kitchen, made a hole in the floor and used a ladder to climb down into the bank branch, where they made another hole down to the vault room.

There, they removed the contents of 99 safe deposit boxes, netting as much as $5 million, according to calculations by the press.

It is estimated, in addition, that the thieves worked all of Sunday morning without being disturbed and then escaped shortly before the bank's alarms sounded.

Investigators say that the job was 'absolutely professional and meticulously planned', and they are convinced, just like prosecutor Viviana Fein, that a 'facilitator' provided the criminals with a secret key so that they could get into the vault itself.

This hypothesis was bolstered when the precision with which the thieves executed the operation was verified, given that they were outfitted with ski masks and gloves, altered the alarm system and modified the bank's motion sensors.

Police do not know with certainty how many thieves entered the bank, but they certainly know of their boastfulness.

'This won't be the robbery of the century, but the one of the millennium,' the thieves wrote on one of the walls of the bank.

The message alludes to the 'robbery of the century,' as the press dubbed the movie-like assault on a Spanish-owned bank on the outskirts of Buenos Aires in 2006.

The haul on that occasion was thought to be around $8 million.

The trials of four of the people accused of that heist, in which the criminals fooled the police by escaping through a drain pipe while they pretended to engage in negotiations, began last month.