All eyes in Chicago on Wednesday were on a man named Alligator Bob, who was paddling around a lagoon in the city searching for – well, you can guess.

Alligators are, safe to say, not a common sighting this far north, and its appearance has both shocked and charmed. The alligator was probably kept as a pet – which is illegal under Illinois law – and then released when it became too large to keep.

“We’ve independently confirmed the alligator is in the lagoon and state reptile specialists say [it’s] between 4 and 5 feet long,” a Chicago police department spokesman, Anthony Guglielmi, tweeted. “The reptile will be humanely trapped tonight and relocated to a zoo for veterinary evaluation.”

Facebook Twitter Pinterest Alligator Bob, a volunteer expert with the Chicago Herpetological Society, sets alligator traps in Humboldt Park Lagoon. Photograph: Ashlee Rezin/Associated Press

By Wednesday, Alligator Bob, a volunteer herpetology expert, was on the case. A handful of agencies are involved in the trapping effort, but Bob has taken the lead because he has so much experience: he has a long history of helping with exotic animal rescues, said Jenny Schlueter, a spokeswoman for Chicago Animal Care and Control. Last year, the volunteer helped rescue a three-foot alligator from the Chicago river. “He’s actually also known as Bob the Snake Man, but these days it’s just Alligator Bob,” says Schlueter. An enigmatic figure, he has declined to give his full name to media.

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“Bob feels that fewer people on the lagoon is better,” says Kelley Gandurski, the executive director of Chicago Animal Care and Control. “We suspect that [he] was raised as a domestic pet, and he’s in unfamiliar waters right now, so he’s not surfacing much.”

People often buy alligators as babies, when they are just 12 inches long – but they grow a foot a year and they can live to be 60 years old and top out at 12ft, said Scott Ballard with the Illinois department of natural resources. “Don’t get one to begin with if you don’t have the facilities to keep it,” he said.

A full-grown animal will need a large lake with an acre of land – hard to come by in a dense city like Chicago. “Alligators are omnivores like humans. They eat veggies and meat. They’ll eat shrimp, venison, they’ll eat all kinds of things,” said Ballard. He added that most captive animals are not aggressive – if one is in captivity, it will come closer to humans to get food. “That doesn’t mean they are coming towards you to attack.”

Kate Chappell (@kchappellnews) There is a 4 foot alligator in Chicago’s Humboldt Park lagoon. That is all... pic.twitter.com/OnvggW01q7

The alligator’s brain is just the size of a pea, and it’s probably terrified, Ballard said. “People who release their pets like this are selfish and irresponsible,” he says. “Instead of finding a zoo or a place to keep them, they dump it – which puts pressure on native species. Plus, if winter comes and this thing isn’t collected, it’ll freeze to death.”

For now, Alligator Bob and the patchwork of agencies are working around the clock to trap the alligator – which appears to be staying beneath the surface as much as possible. “We don’t want people to let their guards down, but it’s interesting to learn that alligators are really scared and nervous and more likely to hide than to attack,” said Gandurski.

The Chicago Tribune on Wednesday published an “exclusive” column from the reptile. “Not cool, people. Not cool at all. What have I done to you?”