An investigation is underway into how 1,000 fish died in a Cardiff river.
The Environment Agency and Cardiff Harbour Authority say tests are being carried out on fish taken from the River Ely and Cardiff Bay.
Initial examinations have shown that the fish - mainly rudd, roach and chub species, have markings that may suggest bacterial infections.
However, other factors such as pollution or a drop in oxygen levels are not being ruled out.
An inquiry into the deaths was started on Monday when samples were taken from the lower reaches of the Ely and in Cardiff Bay itself.
Post mortem examinations on the fish are being carried out at the Environment Agency's national fisheries laboratory.
John Harrison, environment manager for the Taff and Ely area, said: "As soon as we know the cause we will be better placed to decide what measures we can take to reduce further impacts.
"People are urged not to touch or take away any of the dead fish."
Scientists have said that at present the involvement of a fish disease "cannot be ruled out".
Officials said they are continuing to monitor the situation in the river and bay, and a clean-up operation to remove the dead fish is continuing.