Twenty-three dead swans have been discovered close to where eight others were found shot with an air gun last week, bringing the total killed to 31 in just a month.
A wildlife rescue group described finding the first eight birds as "one of the worst incidents" it had ever attended.
The birds had been shot in the head with airgun pellets and were found in a field outside Blackford in the Wedmore area of Somerset.
The Secret World charity said a further 23 carcasses have been found in nearby Westham and Godney.
After reports of the shooting £10,500 has been raised by the local community and is being offered as a reward for information that leads to the conviction of the gunman.
One swan survived the initial attack and was taken to the Secret World rescue centre in East Huntspill, where it is still recovering.
The charity's founder, Pauline Kidner, said: "We are all very upset with the situation.
"More reports of suspicious deaths of swans on the Levels have been reported.
"Secret World is urging the public to remain vigilant when out walking in the local area.
"If anyone sees more than one dead swan then please notify us so we are able to collect evidence and investigate the matter further."
Mrs Kidner added some of the slaughtered swans may have been birds rescued from the freezing ice on a waterway on the Somerset Levels just before Christmas.
Charity spokeswoman, Debbie O'Keefe, said: "It's very unusual to find more than one swan, certainly when you find a group like this it suggests there is something going on we need to know about.
"The first eight swans were clearly shot, but because the swans found since then are so decomposed we are unable to tell the cause of death for sure, but it is likely they were killed about the same time.
"It's very upsetting and the fact this person or these people think they can get away with it - we just need to get as much evidence as possible to make sure they don't, and that's why if anybody comes across even just one bird in the area, they need to contact us."
Since the first shooting it emerged members of the public found a group of dead swans at Wedmore at the end of January but only reported it after they saw reports of the Blackford killings.
Five more dead swans were found at Godney and yesterday another 10 were found by wildlife workers from Secret World.
Speaking after the initial discovery on February 21 Mrs Kidner said one of the swans must "have suffered terribly" after it was shot three times.
The RSPCA said it is already investigating the first incident and are able to prosecute.
An Avon and Somerset Police spokeswoman said the neighbourhood team was carrying out extra patrols.
Secret World is appealing to anyone with information about the incidents to phone on 01278 783250.