Gardai are trawling through records documenting the movement of ambulances following serious allegations of misuse of drugs by paramedics.
The probes by the HSE and the gardai were launched after an ambulance is alleged to have made several unscheduled stops at a west Dublin address.
It is believed a number of staff at the National Ambulance Service in north Dublin were interviewed and allegations of misuse of drugs during working hours were made.
Sources said large volumes of records are being reviewed to establish if the allegations warrant further action.
"Gardai are looking at records showing the movements of ambulances," a source told the Herald. "They are trying to establish patterns and if ambulances were making unscheduled stops.
"The allegations focus around a particular area in the city, so that will be examined."
Paramedic Dave O'Sullivan - the brother of Garda Commissioner Noirin O'Sullivan - has been caught up in the probe, but denies any wrongdoing.
Mr O'Sullivan (50), who lives in north county Dublin, is on leave - along with another worker - pending the outcome of an investigation into the claims, which he strenuously denies.
Mr O'Sullivan told the Sunday World newspaper "there's no truth" in the claims.
It is understood that Mr O'Sullivan has not been suspended from work as a result of the investigation.
"It's pure bulls**t. There's no truth in it," he said.
"Nothing happened at the ambulance base in Swords, that's the problem.
"There's nothing like the health board for rumours. I can't say anymore," he added.
He also said the allegations "could be" motivated by the fact that his sister is the country's Garda Commissioner.
"It could be, I don't know, I just don't know," he said.
Although the HSE would not comment further on the allegations, a HSE spokesman said the organisation had confidence in the National Ambulance Service. "Of course the HSE is confident in the ambulance service," a spokesman said.
"There's adequate services in the National Ambulance Service. An investigation is ongoing at the moment and we can't speculate on the outcome."
Meanwhile, a garda spokesman confirmed the allegations were being investigated.