Tests have been unable to identify "where and when" a group of whooper swans found dead in the Borders ingested the lead which poisoned them.
Sixteen bird carcases were found in the Fairnington area near Kelso between December 2017 and January this year.
South of Scotland MSP Colin Smyth asked Environment Secretary Roseanna Cunningham about the incident.
She said five of the swans had been found to have died from ingesting lead and had not been shot.
"The cause of death for the remaining 11 whooper swans was not identified although, as they were found in exactly the same set of circumstances, it is highly likely they suffered the same fate," she said.
"However, this could not be confirmed during post mortem due to the advanced state of decomposition in most of the carcases.
"It is not possible to say where and when the swans ingested the lead."
Ms Cunningham told Mr Smyth that the area where it might have happened was a large one.
"It is understood the swans migrated from Iceland to the UK for the winter and the lead could have been ingested anywhere on that journey or before, during or after," she said.
"There is no evidence to suggest the swans ingested the lead over a wetland area, although this remains a possibility.
"There is no wildfowling undertaken at the location where they were found."