NEW YORK (CBSNewYork) — There was a mystery brewing in Brooklyn after a bevy of swans suddenly went missing from a lake, with some concerned they didn’t just fly off on their own.

The swans are no longer swimming in Prospect Park, but where they’ve gone is anyone’s guess.

“It’s really heartbreaking when they disappear and we don’t know why,” said Whitney Williams from W.I.L.D. for Prospect Park.

Williams’ group keeps an eye on the park’s abundant wildlife and says it’s hard to believe the species known as mute swans left on their own and seemingly vanished overnight in early June.

“We have never seen an entire flock of swans disappear,” Mary Beth Artz from W.I.L.D. said. “It just doesn’t make sense, it’s not a natural occurrence as far as I’m concerned.”

The group says there’s reason to fear human hands may have intervened after a family of swans was euthanized from Oneida Lake near Syracuse by the USDA on Monday.

The agency confirms two adults and four cygnets were put down, it says humanely, after a kayaker was attacked by one of the birds. It was “an intentional drowning of the baby cygnets,” according to a scathing letter written Wednesday by State Assemblyman Steven Cymbrowitz. In it he accused the state Department of Environmental Conservation of using the USDA to circumvent a two-year moratorium which suspends the killing of the species.

“The respect for that family? There was none,” said Artz. “I mean they could have relocated them.”

The DEC says it can’t speculate on the disappearance of the swans from Prospect Park, and says mute swans are fully capable of flight and may have moved to another location.

In the meantime, fliers have been posted all over the park in hopes that someone might have more information.