Gardaí yesterday interviewed the driver of the bus involved in Tuesday's crash near Clara, Co Offaly, in which a 15-year-old boy died.
Gardaí from the Dublin Castle-based fatal accident investigation team, along with officers from the Garda Technical Bureau, the regional traffic unit and local gardaí, are involved in the investigation. A Garda spokeswoman said there would be a "full and comprehensive" investigation.
The 1989 Mercedes Benz private-hire bus had been bringing more than 30 pupils from Clara to Killina Presentation Secondary School, near Rahan, when it overturned on Wednesday morning. No other vehicle was involved.
It is understood that the owner of the bus, local transport operator Ray McKeown, has given a statement to gardaí.
It also emerged yesterday that Mr McKeown had previously held a contract with Bus Éireann to run buses to two schools in Kilbeggan and Tullamore as part of the State's school transport scheme.
He provided the service from December 2004 onwards, but Bus Éireann withdrew the contract last February because the company was unhappy with details provided by Mr McKeown's company.
A spokeswoman for Bus Éireann said they found the operator "unsatisfactory from an administrative point of view".
"We did take steps to replace Mr McKeown's service with another operator," she said.
Mr McKeown could not be contacted by The Irish Times yesterday.
Minister for Transport Martin Cullen said in the Dáil yesterday that the bus involved in Tuesday's crash was not registered on the bus operator's licence.
He said the owner of the bus was issued with a road transport operator's licence in 2003, which allows him to carry passengers for hire or reward.
"There is one bus registered on the licence but this is not the vehicle involved in yesterday's accident," he said.
The vehicle, a 35-seat bus, was registered for road-tax purposes, but in the name of the bus operator's transport manager. Gardaí would be following this up with officials in his department, Mr Cullen added.
Breaches of passenger licensing regulations, which date back to 1932, normally carry fines of no more than €127.
A total of 32 people were treated in hospital following the crash. One student and the driver of the bus, Ger Buckley, remain in Tullamore hospital. Gardaí have interviewed Mr Buckley in hospital.
The bus was issued with a certificate of roadworthiness last September. It was also visually checked by a Garda public service vehicles (PSV) inspector.