An "extensive investigation" into asbestos found at Harrison School has failed to identify the source of the material.
WorkSafe ACT launched the probe after asbestos mixed with gravel and recycled building materials was discovered in garden beds at the public school in Canberra's north in August 2018.
The discovery and the mystery around who had brought the asbestos to the school raised questions about where else the material might be found and who may have been exposed to it.
The ACT government stressed at the time that the material found at Harrison School was "a small amount" of asbestos and that a licensed assessor had identified it as being non-friable, meaning it was unlikely to become airborne and pose a risk to students and staff unless disturbed.
About a week later, non-friable asbestos was also found in a garden bed at the front of Mother Teresa Catholic Primary School, which is also in Harrison, after its principal Peter Hughes arranged for the garden beds there to be tested.
Responding to questions from the Sunday Canberra Times this week, a WorkSafe ACT spokeswoman said its investigation into the Harrison School discovery had concluded.
"WorkSafe ACT conducted an extensive investigation into the potential sources of contaminated garden material," the spokeswoman said.
"Records of several different landscape contractors were reviewed. In addition, potential sources of recycled material were tested as part of the investigation.
"WorkSafe concluded that there was insufficient evidence to identify the source of the small amount of bonded asbestos material. None of the suppliers' sites that were tested had a positive reading.
"A range of other sites that received material at that time from the suppliers that delivered to the Harrison School were also tested and no contamination was identified."
The asbestos was removed from Harrison School by a licensed assessor last year.
Rachel Stephen-Smith, the minister responsible for the Asbestos Response Taskforce, last week told the ACT Legislative Assembly last week that 69 public schools, including Jervis Bay School, had a "confirmed presence of asbestos-containing materials".
She said the ACT government's education directorate spent about $1.4 million in 2018-19 to undertake 372 hazardous materials testing and removal projects.
The largest of these projects was the removal and remediation of garden beds at Harrison School.
"A licensed asbestos removalist was engaged during the September school holidays and a clearance certificate was issued," Ms Stephen-Smith said.
"Plans for the garden bed rehabilitation were subsequently completed, and landscape reinstatement works commenced in June this year."