Witnesses claim they saw the creature, which is possibly an alligator or a giant snake, drag a 16lb Canada goose under the River Lea fast and without a sound.
They take it as no coincidence that the number of swans on the river and waterways near the £9billion Olympic park in East London is also dropping.
Mike Wells, who saw the goose attack from a boat, told the Sun: “The bird just went vertically down. I was gobsmacked. There was no sign of what took it, but it was obviously pretty big.”
Wells, who lives in the Lea Valley, believes the creature that was seen dragging a goose beneath the Lea in 2005, has re-emerged.
Wildlife experts investigating the 2005 incident, suspected the culprit was an alligator or a giant turtle after finding 8ft long holes burrowed in the bank.
Wells added: “It must be the same creature. Some people I’ve spoken to think it could be a very large pike and I’ve seen some turtles about a foot across, but they’re not really big enough to take a goose.”
Zoology graduate Michael Allen, who lives near the Olympic Park, said: “It might be an escaped pet snake like a python. A small goose or a duck could be a perfect meal.”
A spokesman for British Waterways suspects the creature may be a “giant pike” or mink, a mammal known to attack ducks or small geese.
He added: “The other possibility is a terrapin, but they’re more likely to go for ducks.”