The source of a diesel spillage which blighted a beauty spot in Arbroath might never be known.
The Scottish Environment Protection Agency (Sepa) made the admission following an investigation into reported pollution at Brothock Burn which sparked fears for local wildlife.
The environmental watchdog and Scottish Water were called to investigate after the alarm was raised three weeks ago at the burn which is used by a range of wildlife including otters.
The watercourse runs alongside a popular dog-walking route and there were fears that contaminated water could poison horses further downstream.
Angus South MSP Graeme Dey said: “This has been a deeply concerning and unsatisfactory episode — deeply concerning because pollution of this nature is a serious matter and unsatisfactory because we seem to have ended up with more questions than answers and are none the wiser as to whether the spill was accidental or deliberate.”
Sepa is continuing discussions with Angus Council and Scottish Water in an effort to determine the source of the incident and to mitigate any impact of the incident.
A joint site visit was made last week and it was confirmed that there is no ongoing discharge.
A spokesman for Sepa said: “Scottish Water have confirmed that they have again checked all of their inputs into the culverted watercourse and also all the Angus Council inputs and no signs of oil has been found.
“Samples have been taken by Scottish Water from the storm tanks serving part of the industrial estate and at the outfall although sample results won’t be known until next week.”
Given that the source has not been detected within the Scottish Water or Angus Council networks, SEPA has also met representatives of RM Condor on the outskirts of Arbroath.
Surface water leaving the miltary site was checked earlier in the investigation but SEPA has since checked drainage drawings and carried out a site visit in the restricted access area.
Fly-tipping was evident in the watercourse near the site boundary but no evidence of oil was detected.
RM Condor representatives said the rubbish would be removed and they will monitor the area for any further dumping.
A Sepa spokesman added: “At present it appears that unfortunately it may not be possible to confirm the source of the incident.”