The first birds were discovered on February 21. Secret World Wildlife Rescue, in East Huntspill, received a call that morning telling them that there were three dead swans in a field.
When rescuers got there they found a total of eight dead birds in two areas, as well as an injured one. The Burnham Area Rescue Boat helped capture the wounded swan and it was taken to the centre to recuperate.
X-rays showed the animals had been killed by air gun pellets.
"One of the swans had been shot three times and must have suffered terribly," states information on the organisation's website
More dead swans were found later. Due to the level of decomposition it was not possible to determine how some of them had died.
Secret World believes some of the swans may have been ones they worked hard to save from the ice around Christmas time.
The centre stated
that a total of 30 swans had been reported as of March 2, although several news outlets have put the number at 31.
On Wednesday it was announced that author Sir Terry Pratchett added £10,000 to the reward money raised by the community.
"It's wanton destruction of a living creature," BBC News
quoted the author of the Discworld novels as saying.
"People who do this sort of thing to an animal are probably a danger to people as well.
"I don't think the punishment will fit the crime.
"For some idiot to fire an airgun for fun - I was just incensed. I've seen the sanctuary, I've lived in Somerset and I know the area."
It's amazing to see that people are so enraged about the killings that they are coming forward and donating very large sums of money," The Daily Mail
quoted Sarah Cowen, animal care manager at Secret World, as saying.
'It's always difficult with cases like this to get people to talk. If someone is carrying out this action they are not likely to be a very nice people and others might be afraid of retribution, but this might make someone more willing to come forward."
Anyone convicted of attacking swans can be sentenced to six months in prison and fined £5,000.