An investigation was launched after six swans were found beheaded and stabbed in various parks in south-east London.

The Royal Society for the Prevention of Cruelty to Animals (RSPCA) began the investigation after two of the six beheaded swans were found in Birchmere Park in the city last month, while another was discovered at Southmere Lake.

Sky News reported, Wednesday, three more cygnets in the area also seemed to have suffered a similar fate after they were reported missing.

RSPCA inspector Nick Wheelhouse said: "Park wardens have informed me that they're aware of the deaths of three swans, three cygnets and one Canada goose so far in recent weeks. It's really concerning because the birds seem to have been stabbed and then beheaded, with their bodies left beside the water."

"While it's a serious animal welfare concern it's also really upsetting for any of the park users who make the grisly find," Wheelhouse added. Exact dates of when the birds were found were not provided by park rangers, the BBC said.

The BBC reported that it is illegal to harm the mute swans which are considered property of the Crown. It also cited the Wildlife and Countryside Act 1981, which said that anyone prosecuted for deliberately trying to injure, kill or take a wild bird could face up to six months in prison and a potential fine of up to £5,000 (around $6,700).

Locals set up 24-hour patrols in an attempt to catch the killers.

Danny Mercer, an organizer of Thamesmead Swan Patrol, said the group were taking shifts to supervise Southmere and Birchmere Lakes.

"Over the past few weeks around ten birds have been killed in and around the Thamesmead area. Dead swans have been found in numerous places around Southmere lake at first the locals thought it was foxes catching them. Then, earlier this week two swans were found gutted and butchered for their breast meat and the rest of the body's laid strewn on the side of the lake. We knew then something was wrong," Mercer said.

Paul West a member of the local swan protection patrol said: "This is an act of wanton cruelty and evil. It is beyond belief. We keep in the shadows and use torches only when we need to. So we try to keep a low profile in the attempt that we catch them in the act."

Some, reports said, were also concerned the killings may be linked to the so-called Corydon cat-killer. More than 360 deaths of cats, foxes and rabbits were recorded in Greater London since September 2015, and the animals had their heads and limbs removed in most cases.

However, an RSPCA spokeswoman said the swan deaths were not believed to be linked to the cat-killer but the possibility had not been ruled out entirely.

However, workers at South Norwood Animal Rescue Liberty (SNARL) thought the method in which the swans died was similar to the pattern seen in many of the cat deaths.