Investigators suspect a known criminal gang specialised in high-stake robberies is behind the attempted tunnel heist.

Police sources said it could not be excluded that foreigners had been recruited for the job, which saw a gang of criminals dig a 15-metre long tunnel from a garage complex in the direction of the subterranean vault of the St Andrew’s HSBC branch.

The criminals were only a few metres shy of their target when it appears the tunnel was abandoned some time ago, the sources added.

Some of the equipment found inside and the metal beams used to support the tunnel ceiling were rusted but it is hard to establish when the would-be thieves were last at work, given the conditions of the tunnel, particularly the humidity levels.

The police sprung into action some months ago following a tip-off although the information they were given originally did not specify which bank was being targeted.

Eventually, investigators located the garage and mounted a surveillance operation lasting weeks but there was no sign of the criminals. It was then decided to raid the garage on grounds of safety because of the possibility of the tunnel collapsing if it flooded with winter rain.

Police Inspector Sandro Camilleri told this newspaper on Monday that the police could not have waited for the robbers to continue digging the tunnel to nab them in the act as such action could constitute a form of entrapment.

But this opinion was contradicted yesterday by Assistant Police Commissioner Pierre Calleja, head of the Criminal Investigation Department.

He insisted that the law gave investigators the faculty to stake out the activity and even perform an arrest as the thieves made their way into the vault.

There could not be entrapment without having an agent provocateur and, in this case, it was very clear that the police had not solicited the perpetrators to dig the tunnel.

“This is permissible at law and there is also jurisprudence to this effect,” Mr Calleja told Times of Malta, referring to a 1990 Criminal Appeals Court judgment handed down against Horace Spiteri, among others. In the judgment, the court justified police action taken to make an arrest related to drug trafficking and possession.

He added that section 346, sub-section 2, of the Criminal Code states that “where authorised by law and in the manner so provided, the police may delay its immediate intervention for the prevention of the commission of an offence”.

Mr Calleja would not explain why the police did not wait longer before acting, citing operational sensitivity.

However, he confirmed that investigators had been monitoring the garage for weeks.

Beyond the investigation itself, the next headache concerns the safety of the tunnel.

Architects have certified the construction as safe for the time being but action will have to be taken in the coming months to secure the area, especially given the damage that could be done by rainwater.