An investigation which led to the arrest of three gardai was sparked after allegations emerged that officers were using "off the book" informants, the Herald has learned.
As part of the probe, "chatter" between notorious criminals and members of the force was picked up on bugged mobile phones.
One of the three - a garda superintendent - was arrested on suspicion of "directly leaking" key information to a criminal gang about a Criminal Assets Bureau (CAB) investigation into its activities.
The gang is understood to have taken advantage of "turbulence" in the officer's personal life which led to him allegedly providing it with information about the probe for personal gain.
An inspector who also was arrested yesterday was held on suspicion of drugs offences linked to the same gang, before he was released without charge.
The officer was suspended from duty upon his release and a file will now be prepared for the DPP.
A detective garda also detained was previously arrested by officers investigating a different aspect of the same probe.
Both this garda and the superintendent had their periods of detention further extended last night.
There are no expectations of further arrests of gardai in the investigation, which has been going on for more than a year.
A file on the three officers and a civilian arrested in the probe last January will now be completed for the DPP.
The investigation began with allegations that gardai in the district where the arrested officers are based were running informants "off the book", a senior source has revealed.
This came to the attention of specialist garda units who are understood to have picked up "chatter" on bugged mobile devices between criminals linked to the gang and gardai in the south of the country.
The information was so serious that a number of specialist units, including the National Bureau of Criminal Investigation and the Drugs and Organised Crime Bureau, became involved in the top secret probe.
In the first phase of the investigation, a garda was suspended from duty last October and later arrested after he was suspected of providing bogus insurance certificates and driving licence documentation to the gang.
Then in January, another garda and a civilian were arrested after specialist officers received information that the garda had been paid €20,000 in return for information on the progress of a CAB investigation into the gang's money-laundering operations.
Despite concerns that information was being leaked to the gang, the CAB still targeted the mob in a series of raids in March.
It seized 115 cars, €43,000 and £1,000 sterling as part of the investigation into drugs money laundering and also froze a bank account containing €30,000.
All three gardai arrested yesterday have vast experience and have been involved in dozens of major investigations.
The detective garda - the most junior of the three, who were all arrested at their homes at 8am yesterday - is being questioned about conspiracy to pervert the course of justice.
He is suspected of attempting to use his job to help the activities of the crime gang.
The separate allegation that he was paid a €20,000 bribe by the gang was secretly recorded when the alleged transaction happened last December, but this was not what he was questioned about yesterday.
Deputy Commissioner John Twomey confirmed yesterday that the gardai had been detained for questioning arising from suspicion they had been involved in the commission of one or more criminal offences.
"An Garda Siochana is fully committed to investigating any alleged wrongdoing or corruption involving garda personnel," he said.